staging: mt7621-gpio: avoid one level indentation in interrupt handler

There is no need to check for 'pending' before loop over the
interrupts using 'for_each_set_bit' if nothing is set the
return values will be the same so just avoid this check avoiding
also one level intentation and improving readability.

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