staging: mt7621-gpio: use MTK_BANK_WIDTH instead of magic number

There are some places where magic number '32' is being used to get
the gpio bank. There already exist a definition MTK_BANK_WIDTH
with this value, so just use it instead.

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