smc91x: let smc91x work well under netpoll

The NETPOLL requires that interrupts remain disabled in its callbacks.

Using *_irq_save()/irq_restore() to replace *_irq_disable()/irq_enable()
functions in NETPOLL's callbacks of smc91x, so that it doesn't enable
interrupts when already disabled, and kgdboe/netconsole would work
properly over smc91x.

Signed-off-by: Dongdong Deng <dongdong.deng@windriver.com>
Acked-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: David S. Miller <davem@davemloft.net>