Input: gpio_keys - fix warning regarding uninitialized 'irq' variable

Commit f2d347ff ("Input: gpio_keys - add device tree support for
interrupt only keys") caused the following build warning:

drivers/input/keyboard/gpio_keys.c: In function 'gpio_keys_probe':
drivers/input/keyboard/gpio_keys.c:647:15: warning: 'irq' may be used uninitialized in this function [-Wmaybe-uninitialized]
drivers/input/keyboard/gpio_keys.c:622:7: note: 'irq' was declared here

Move button->irq initialization into proper branch and get rid of the
temporary.

Reported-by: Olof's autobuilder <build@lixom.net>
Reported-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>