axnet_cs: use spin_lock_irqsave in ax_interrupt

Use spin_lock_irqsave instead of spin_lock in ax_interrupt because
the interrupt handler can also be invoked from ei_watchdog.

Signed-off-by: Ken Kawasaki <ken_kawasaki@spring.nifty.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>