ARM: OMAP: Fix GPIO IRQ mask handling

The GPIO IRQ mask was retrieved incorrectly in cases where we have a mask
register instead of an enable register. Also we should only return the
valid bits depending on the bank size.

This fixes a bug on 1510/1610 based OMAPs where GPIO IRQs are not
delivered.

Signed-off-by: Imre Deak <imre.deak@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>