Sometimes host->clock could be zero which is a legal situation. This
patch checks host->clock before usage as a denominator when timeout is
calculated. A similar patch is applied for mmc core (see commit e9b86841,
"mmc: fix division by zero in MMC core").

Without this patch, the execution of the sdhci_calc_timeout could end up
with a backtrace:

<0>[    4.014319] divide error: 0000 [#1] PREEMPT SMP
<4>[    4.014352] Modules linked in: g_ether
<4>[    4.014376]
<4>[    4.014393] Pid: 33, comm: kworker/u:2 Not tainted 3.0.0+ #646
<4>[    4.014421] EIP: 0060:[<c12fa38e>] EFLAGS: 00010046 CPU: 1
<4>[    4.014449] EIP is at sdhci_calc_timeout+0x2e/0x100
<4>[    4.014468] EAX: 00000000 EBX: f5930fc8 ECX: 00000000 EDX: 00000000
<4>[    4.014488] ESI: f5291de8 EDI: f5291db8 EBP: f5291c6c ESP: f5291c50
<4>[    4.014508]  DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0068
<0>[    4.014529] Process kworker/u:2 (pid: 33, ti=f5290000 task=f53065a0 task.ti=f5290000)
<0>[    4.014546] Stack:
<4>[    4.014557]  00000082 c1054fdd f5291c78 04000000 f5930fc8 f5291de8 f5291db8 f5291cac
<4>[    4.014611]  c12fab7c c107a98b f5291c88 c13b6d3f f593109c f5882000 f5291cac c1054fdd
<4>[    4.014663]  00000000 00000000 f5882000 00000082 f5930fc8 f5291db8 0000000a f5291ccc
<0>[    4.014716] Call Trace:
<4>[    4.014743]  [<c1054fdd>] ? mod_timer+0x11d/0x380
<4>[    4.014770]  [<c12fab7c>] sdhci_prepare_data+0x2c/0x3a0
<4>[    4.014798]  [<c107a98b>] ? trace_hardirqs_off+0xb/0x10
<4>[    4.014827]  [<c13b6d3f>] ? _raw_spin_unlock_irqrestore+0x2f/0x60
<4>[    4.014854]  [<c1054fdd>] ? mod_timer+0x11d/0x380
<4>[    4.014880]  [<c12fc7db>] sdhci_send_command+0xdb/0x210
<4>[    4.014906]  [<c12fd5f3>] sdhci_request+0xc3/0x150
<4>[    4.014932]  [<c12ec56a>] mmc_start_request+0xda/0x200
<4>[    4.014960]  [<c120d7c2>] ? __raw_spin_lock_init+0x32/0x60
<4>[    4.014989]  [<c1066a85>] ? __init_waitqueue_head+0x35/0x50
<4>[    4.015015]  [<c12ec70b>] mmc_wait_for_req+0x7b/0x90
<4>[    4.015045]  [<c12f0c67>] mmc_send_cxd_data+0xf7/0x130
<4>[    4.015076]  [<c12ecbc0>] ? mmc_erase+0x140/0x140
<4>[    4.015102]  [<c12f139d>] mmc_send_ext_csd+0x1d/0x20
<4>[    4.015125]  [<c12efef0>] mmc_get_ext_csd+0x70/0x140
<4>[    4.015151]  [<c12effe8>] mmc_compare_ext_csds+0x28/0x190
<4>[    4.015176]  [<c12f039f>] mmc_init_card+0x24f/0x650
<4>[    4.015201]  [<c13b6d5d>] ? _raw_spin_unlock_irqrestore+0x4d/0x60
<4>[    4.015226]  [<c107fd9c>] ? trace_hardirqs_on_caller+0x11c/0x160
<4>[    4.015255]  [<c12f09a4>] mmc_attach_mmc+0xa4/0x190
<4>[    4.015282]  [<c12ee3f0>] mmc_rescan+0x210/0x240
<4>[    4.015311]  [<c105f9b6>] process_one_work+0x176/0x550
<4>[    4.015336]  [<c105f93a>] ? process_one_work+0xfa/0x550
<4>[    4.015360]  [<c12ee1e0>] ? mmc_init_erase+0x140/0x140
<4>[    4.015385]  [<c1061c2a>] worker_thread+0x12a/0x2c0
<4>[    4.015410]  [<c1061b00>] ? manage_workers.clone.18+0x100/0x100
<4>[    4.015437]  [<c1066244>] kthread+0x74/0x80
<4>[    4.015463]  [<c10661d0>] ? __init_kthread_worker+0x60/0x60
<4>[    4.015490]  [<c13b7dfa>] kernel_thread_helper+0x6/0xd
<0>[    4.015507] Code: 57 89 d7 56 53 89 c3 83 ec 10 8b 40 04 8b 72 28 f6 c4 10 89 45 f0 0f 85 91 00 00 00 85 f6 0f 84 c1 00 00 00 8b 4e 04 31 d2 89 c8 <f7> 73 58 ba d3 4d 62 10 89 c1 8b 06 f7 e2 c1 ea 06 01 d1 f7 45
<0>[    4.015829] EIP: [<c12fa38e>] sdhci_calc_timeout+0x2e/0x100 SS:ESP 0068:f5291c50
Reported-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Chris Ball <cjb@laptop.org>