Input: gpio_keys - verify that supplied GPIO numbers are valid

As David Brownell pointed out, gpio_keys driver does not check
return code of gpio_to_irq().

This patch adds the gpio_to_irq return code check to gpio_keys
and moves the IRQ edge type setting to request_irq flags to avoid
changing the irq type before we have confirmed we can use it.

Signed-off-by: Anti Sullin <anti.sullin@artecdesign.ee>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>