Input: gpio_keys - seperate individual button setup to make code neater

Move the code that deals with setting up each individual button out into
a new function to reduce the indentation and allow us to common up some
of the error recovery code.

Signed-off-by: Ben Dooks <ben@simtec.co.uk>
Signed-off-by: Simtec Linux Team <linux@simtec.co.uk>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>