Instead of returning EBUSY when getting a command while another one is
running, we queue them. Upon completion of the pending command, the next
one is processed.
Besides the fact that it simplifies the pn533 locking scheme, it also
comes with the nice side effect of fixing the following warning:

[   82.274297] =====================================
[   82.274297] [ BUG: bad unlock balance detected! ]
[   82.274298] 3.5.0-rc1+ #1 Not tainted
[   82.274299] -------------------------------------
[   82.274300] kworker/u:1/16 is trying to release lock (&dev->cmd_lock) at:
[   82.274305] [<ffffffff8144f246>] mutex_unlock+0x9/0xb
[   82.274305] but there are no more locks to release!
[   82.274306]
[   82.274306] other info that might help us debug this:
[   82.274306] 2 locks held by kworker/u:1/16:
[   82.274311]  #0:  (pn533){.+.+..}, at: [<ffffffff8103a67d>]
+process_one_work+0x145/0x2e2
[   82.274314]  #1:  ((&dev->cmd_work)){+.+...}, at: [<ffffffff8103a67d>]
+process_one_work+0x145/0x2e2
[   82.274314]
[   82.274314] stack backtrace:
[   82.274315] Pid: 16, comm: kworker/u:1 Not tainted 3.5.0-rc1+ #1
[   82.274315] Call Trace:
[   82.274317]  [<ffffffff8144f246>] ? mutex_unlock+0x9/0xb
[   82.274321]  [<ffffffff81059841>] print_unlock_inbalance_bug+0xda/0xe4
[   82.274323]  [<ffffffff8105c74c>] lock_release_non_nested+0xb2/0x232
[   82.274325]  [<ffffffff8105a61e>] ? mark_held_locks+0x6d/0x95
[   82.274326]  [<ffffffff8144f246>] ? mutex_unlock+0x9/0xb
[   82.274328]  [<ffffffff81451105>] ? _raw_spin_unlock_irqrestore+0x40/0x5c
[   82.274329]  [<ffffffff8144f246>] ? mutex_unlock+0x9/0xb
[   82.274330]  [<ffffffff8105ca42>] lock_release+0x176/0x1ac
[   82.274333]  [<ffffffff8123de14>] ? pn533_send_complete+0xa8/0xa8
[   82.274334]  [<ffffffff8144f1d6>] __mutex_unlock_slowpath+0xb0/0x117
[   82.274336]  [<ffffffff8144f246>] mutex_unlock+0x9/0xb
[   82.274337]  [<ffffffff8123de65>] pn533_wq_cmd_complete+0x51/0x55
[   82.274338]  [<ffffffff8103a6db>] process_one_work+0x1a3/0x2e2
[   82.274340]  [<ffffffff8103a67d>] ? process_one_work+0x145/0x2e2
[   82.274341]  [<ffffffff8103b119>] worker_thread+0xcf/0x153
[   82.274343]  [<ffffffff8103b04a>] ? manage_workers.isra.22+0x16b/0x16b
[   82.274344]  [<ffffffff8103b04a>] ? manage_workers.isra.22+0x16b/0x16b
[   82.274346]  [<ffffffff8103eb11>] kthread+0x95/0x9d
[   82.274348]  [<ffffffff81452ef4>] kernel_thread_helper+0x4/0x10
[   82.274351]  [<ffffffff81046561>] ? finish_task_switch+0x45/0xc3
[   82.274352]  [<ffffffff814514f0>] ? retint_restore_args+0x13/0x13
[   82.274353]  [<ffffffff8103ea7c>] ? __init_kthread_worker+0x55/0x55
[   82.274354]  [<ffffffff81452ef0>] ? gs_change+0x13/0x13
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>