staging: mt7621-gpio: use module_platform_driver() instead subsys initcall

The driver's init function don't do anything besides registering the platform
driver, and the exit function which is not included in the driver should only
do driver unregister. Because of this module_platform_driver() macro could
just be used instead of having separate functions.

Currently the macro is not being used because the driver is initialized at
subsys init call level but this isn't necessary since platform devices are
defined in the DT as dependencies so there's no need for init calls order.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>