[PATCH] smc91x: plug race between TX tasklet and driver reset

The race causes a kernel oops when smc_hardware_send_pkt() tries to
dereference pending_tx_skb which would have been freed from one of the
driver reset paths just after the tx_task tasklet has been scheduled.
This race is possible on SMP but was uncovered by the kernel RT work.

Signed-off-by: Nicolas Pitre <nico@cam.org>