smc911x: Make the driver safer on SMP

This patch extends the critical regions covered by lp->lock to make it
safer on SMP. The main failure point was the smc911x_hard_start_xmit
function being called from different CPUs. It was tested on the ARM SMP
platforms.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>