net: Fix ks8851 snl NULL pointer dereference Oops

This fixes the NULL pointer dereference Oops in the ks8851 snl network
driver during transmission.  All socket buffers from the queue are
processed inside the loop, but new workqueues have been scheduled to
run.

Signed-off-by: Tristram Ha <Tristram.Ha@micrel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>