net: ks8851: Fix deadlock with the SPI chip variant

In the Linux kernel, the following vulnerability has been resolved:

net: ks8851: Fix deadlock with the SPI chip variant

When SMP is enabled and spinlocks are actually functional then there is
a deadlock with the 'statelock' spinlock between ks8851_start_xmit_spi
and ks8851_irq:

watchdog: BUG: soft lockup - CPU#0 stuck for 27s!
call trace:
queued_spin_lock_slowpath+0x100/0x284
do_raw_spin_lock+0x34/0x44
ks8851_start_xmit_spi+0x30/0xb8
ks8851_start_xmit+0x14/0x20
netdev_start_xmit+0x40/0x6c
dev_hard_start_xmit+0x6c/0xbc
sch_direct_xmit+0xa4/0x22c
__qdisc_run+0x138/0x3fc
qdisc_run+0x24/0x3c
net_tx_action+0xf8/0x130
handle_softirqs+0x1ac/0x1f0
__do_softirq+0x14/0x20
____do_softirq+0x10/0x1c
call_on_irq_stack+0x3c/0x58
do_softirq_own_stack+0x1c/0x28
__irq_exit_rcu+0x54/0x9c
irq_exit_rcu+0x10/0x1c
el1_interrupt+0x38/0x50
el1h_64_irq_handler+0x18/0x24
el1h_64_irq+0x64/0x68
__netif_schedule+0x6c/0x80
netif_tx_wake_queue+0x38/0x48
ks8851_irq+0xb8/0x2c8
irq_thread_fn+0x2c/0x74
irq_thread+0x10c/0x1b0
kthread+0xc8/0xd8
ret_from_fork+0x10/0x20

This issue has not been identified earlier because tests were done on
a device with SMP disabled and so spinlocks were actually NOPs.

Now use spin_(un)lock_bh for TX queue related locking to avoid execution
of softirq work synchronously that would lead to a deadlock.