Fix NULL pointer dereference in ks8851_tx_work by checking if dequeued
list is already empty before writing the packet to TX FIFO

 Unable to handle kernel NULL pointer dereference at virtual address 00000050
 PC is at ks8851_tx_work+0xdc/0x1b0
 LR is at wait_for_common+0x148/0x164
 pc : [<c01c0df4>]    lr : [<c025a980>]    psr: 20000013
 Backtrace:
  ks8851_tx_work+0x0/0x1b0
  worker_thread+0x0/0x190
  kthread+0x0/0x90
Signed-off-by: NAbraham Arce <x0066660@ti.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>