bcma: prevent irq handler from firing when registered

With this patch we prevent the irq from being fired when it is
registered. The Hardware fires an IRQ when input signal XOR polarity
AND gpio mask is 1. Now we are setting polarity to a vlaue so that is
is 0 when we register it.

In addition we also set the irq mask register to 0 when the irq handler
is initialized, so all gpio irqs are masked and there will be no
unexpected irq.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Tested-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6304/