Patch from Deepak Saxena

Supersedes 2226/1

According to the IXP Programmer's Reference Manual, a read from
IRQ_STATUS can only return '1' for IRQ sources that have been
explicitly enabled in IRQ_ENABLE. So if we never enable 'invalid'
IRQ sources, we don't actually have to mask off IRQ_STATUS with
IXP2000_VALID_IRQ_MASK.

Signed-off-by: Deepak Saxena
Signed-off-by: Russell King