MIPS: JZ4740: GPIO: Simplify IRQ demuxer

We already know the base IRQ for a GPIO chip, so there is no need to
recalculate it in the demux handler.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Cc: Lars-Peter Clausen <lars@metafoo.de>
Patchwork: http://patchwork.linux-mips.org/patch/2432/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>