Input: gpio_keys_polled - defer probing if GPIO probing is deferred

If GPIO probing is deferred, the driver tries to claim an invalid GPIO line
which leads to an error message like this:

  gpio-keys-polled buttons.2: unable to claim gpio 4294966779, err=-22
  gpio-keys-polled: probe of buttons.2 failed with error -22

We should make sure that error code returned by of_get_gpio_flags (including
-EPROBE_DEFER) is propagated up the stack.

Cc: stable@vger.kernel.org
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>