staging: mt7621-gpio: change lock place in irq mask and unmask functions

Functions mediatek_gpio_irq_umask mediatek_gpio_irq_unmask are
reading and modifying registers but only the write is being hold.
It should be a complete lock instead for those which are type of
"read-modify-write". This makes more sense.

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