gpio: 74xx-mmio: remove unneeded platform_set_drvdata() call

The platform_set_drvdata() was needed when the driver had an explicit
remove function.
That function got removed a while back, so we don't need to keep a pointer
(on 'dev->driver_data') for the private data of the driver anymore.

Signed-off-by: Alexandru Ardelean <alex@shruggie.ro>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>