Blackfin: do not error if GPIO IRQ is requested already as GPIO

Some drivers expect to be able to request both as GPIO and GPIO IRQ, so
allow that use case.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>