mac8390: propagate error code from request_irq

Use the request_irq() error code as the return value for mac8390_open().
EAGAIN doesn't make sense for Nubus slot IRQs. Only this driver can claim
this IRQ (until the NIC is removed, which means everything is powered
down).

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>