gpio: generic: fix GPIO_GENERIC_PLATFORM is set to module case

GPIO_GENERIC_PLATFORM is a tristate. If the module option is
selected the resulting gpio-generic.ko will lack most of the
module initialzation and probe code.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>