regulator: max77686: Use module_platform_driver() instead subsys initcall

regulator: max77686: Use module_platform_driver() instead subsys initcall

The driver's init and exit function don't do anything besides registering
and unregistering the platform driver, so the 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 consumer devices are
defined in the DT as dependencies so there's no need for init calls order.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>