staging: mt7621-gpio: change 'to_mediatek_gpio' to make just a one line return

Function to_mediatek_gpio can directly return without declaring
anything else in its body improving readability. Also change
pointer '*' declaration to be with return type in the upper line.

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