forcedeth: spin_unlock_irq in interrupt handler fix

The replacement of spin_lock_irq/spin_unlock_irq pair in interrupt
handler by spin_lock_irqsave/spin_lock_irqrestore pair.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Denis Efremov <yefremov.denis@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>