Input: gpio_keys - convert to use devm_*

This makes the error handling much more simpler than open-coding everything
and in addition makes the probe function smaller an tidier.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>