Fixes the following NULL pointer dereference on PHY connect error path
teardown:

[    2.291010] sja1105 spi0.1: Probed switch chip: SJA1105T
[    2.310044] sja1105 spi0.1: Enabled switch tagging
[    2.314970] fsl-gianfar soc:ethernet@2d90000 eth2: error -19 setting up slave phy
[    2.322463] 8<--- cut here ---
[    2.325497] Unable to handle kernel NULL pointer dereference at virtual address 00000018
[    2.333555] pgd = (ptrval)
[    2.336241] [00000018] *pgd=00000000
[    2.339797] Internal error: Oops: 5 [#1] SMP ARM
[    2.344384] Modules linked in:
[    2.347420] CPU: 1 PID: 64 Comm: kworker/1:1 Not tainted 5.5.0-rc5 #1
[    2.353820] Hardware name: Freescale LS1021A
[    2.358070] Workqueue: events deferred_probe_work_func
[    2.363182] PC is at kthread_destroy_worker+0x4/0x74
[    2.368117] LR is at sja1105_teardown+0x70/0xb4
[    2.372617] pc : [<c036cdd4>]    lr : [<c0b89238>]    psr: 60000013
[    2.378845] sp : eeac3d30  ip : eeab1900  fp : eef45480
[    2.384036] r10: eef4549c  r9 : 00000001  r8 : 00000000
[    2.389227] r7 : eef527c0  r6 : 00000034  r5 : ed8ddd0c  r4 : ed8ddc40
[    2.395714] r3 : 00000000  r2 : 00000000  r1 : eef4549c  r0 : 00000000
[    2.402204] Flags: nZCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment none
[    2.409297] Control: 10c5387d  Table: 8020406a  DAC: 00000051
[    2.415008] Process kworker/1:1 (pid: 64, stack limit = 0x(ptrval))
[    2.421237] Stack: (0xeeac3d30 to 0xeeac4000)
[    2.612635] [<c036cdd4>] (kthread_destroy_worker) from [<c0b89238>] (sja1105_teardown+0x70/0xb4)
[    2.621379] [<c0b89238>] (sja1105_teardown) from [<c10717fc>] (dsa_switch_teardown.part.1+0x48/0x74)
[    2.630467] [<c10717fc>] (dsa_switch_teardown.part.1) from [<c1072438>] (dsa_register_switch+0x8b0/0xbf4)
[    2.639984] [<c1072438>] (dsa_register_switch) from [<c0b89c30>] (sja1105_probe+0x2ac/0x464)
[    2.648378] [<c0b89c30>] (sja1105_probe) from [<c0b11a5c>] (spi_drv_probe+0x7c/0xa0)
[    2.656081] [<c0b11a5c>] (spi_drv_probe) from [<c0a26ab8>] (really_probe+0x208/0x480)
[    2.663871] [<c0a26ab8>] (really_probe) from [<c0a26f0c>] (driver_probe_device+0x78/0x1c4)
[    2.672093] [<c0a26f0c>] (driver_probe_device) from [<c0a24c48>] (bus_for_each_drv+0x80/0xc4)
[    2.680574] [<c0a24c48>] (bus_for_each_drv) from [<c0a26810>] (__device_attach+0xd0/0x168)
[    2.688794] [<c0a26810>] (__device_attach) from [<c0a259d8>] (bus_probe_device+0x84/0x8c)
[    2.696927] [<c0a259d8>] (bus_probe_device) from [<c0a25f24>] (deferred_probe_work_func+0x84/0xc4)
[    2.705842] [<c0a25f24>] (deferred_probe_work_func) from [<c03667b0>] (process_one_work+0x22c/0x560)
[    2.714926] [<c03667b0>] (process_one_work) from [<c0366d8c>] (worker_thread+0x2a8/0x5d4)
[    2.723059] [<c0366d8c>] (worker_thread) from [<c036cf94>] (kthread+0x150/0x154)
[    2.730416] [<c036cf94>] (kthread) from [<c03010e8>] (ret_from_fork+0x14/0x2c)

Checking for NULL pointer is correct because the per-port xmit kernel
threads are created in sja1105_probe immediately after calling
dsa_register_switch.

Fixes: a68578c2 ("net: dsa: Make deferred_xmit private to sja1105")
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>