[netdrvr] convert sbmac tx to spin_lock_irqsave to prevent early IRQ enable

Netpoll will call the interrupt handler with interrupts
disabled when using kgdboe, so spin_lock_irqsave() should
be used instead of spin_lock_irq() to prevent interrupts
from being incorrectly enabled.

Signed-off-by: Weiwei Wang <weiwei.wang@windriver.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>