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

Commit af906faa ("Input: gpio_keys - fix warning regarding uninitialized
'irq' variable") introduced the following build warning:

drivers/input/keyboard/gpio_keys.c:625:16: warning: 'button' may be used uninitialized in this function [-Wmaybe-uninitialized]

Move the 'button' initialization to a proper location to avoid such warning.

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