Blackfin: bf537: fix excessive gpio int demuxing

The search logic in the gpio demux walks all possible gpio blocks starting
at the specified pin.  The trouble on bf537 parts when we demux the port
F and port G mask A interrupts is that we also demux port H mask A ints.
Most of the time this isn't an issue as people don't usually use port H,
but might as well avoid it when possible.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>