tty: serial: 8250_mtk: use pm_runtime callbacks for enabling

The pm_runtime callbacks already enable and disable the device.
Use them in probe() and remove() instead of duplicating the
code. This allows us to concentrate more code for enabling/disabling
the UART in a single place.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>