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.

Fixes: 3dc5d445

 ("net: ks8851: Fix TX stall caused by TX buffer overrun")
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Simon Horman <horms@kernel.org>
Cc: netdev@vger.kernel.org
Cc: stable@vger.kernel.org # 5.10+
Signed-off-by: Ronald Wahl <ronald.wahl@raritan.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20240706101337.854474-1-rwahl@gmx.de
Signed-off-by: Paolo Abeni <pabeni@redhat.com>