serial: mctrl_gpio: export mctrl_gpio_disable_ms and mctrl_gpio_init

To be able to make use of the mctrl-gpio helper from a module these
functions must be exported. This was forgotten in the commit introducing
support interrupt handling for these functions (while it was done for
mctrl_gpio_enable_ms, *sigh*).

Fixes: ce59e48f ("serial: mctrl_gpio: implement interrupt handling")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>