davinci: gpio irq enable tweaks

davinci: gpio irq enable tweaks

Fix two IRQ triggering bugs affecting GPIO IRQs:

 - Make sure enabling with IRQ_TYPE_NONE ("default, unspecified")
   isn't a NOP ... default to both edges, at least one must work.

 - As noted by Kevin Hilman, setting the irq trigger type for a
   banked gpio interrupt shouldn't enable irqs that are disabled.

Since GPIO IRQs haven't been used much yet, it's not clear these
bugs could have affected anything.  The few current users don't
seem to have been obviously suffering from these issues.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>