Blackfin: bf54x: don't ack GPIO ints when unmasking them

When unmasking a GPIO interrupt on a BF54x part, the code will implicitly
ack any pending interrupts.  This is not what unmasking should do and can
cause people to miss interrupts from their devices, so punt the code.

Reported-by: Rutger Hofman <rutger@cs.vu.nl>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>