staging: mt7621-gpio: remove no more necessary PIN_MASK macro

PIN_MASK macro was being used because of the fact we were only
using one interrupt controller for all of the gpio chips. This
has been changed to use one per gpio chip and each has 32 irqs.
Because of this this macro is not needed anymore. Use BIT macro
instead.

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