staging: mt7621-gpio: avoid check for NULL in 'to_mediatek_gpio' calls

Function 'to_mediatek_gpio' cannot return NULL, so this NULL
checkings are pointless.

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