staging: mt7621-gpio: avoid to set up irqs if not defined in dts

If there is no interrupt defined in the dts 'irq_of_parse_and_map'
returns 0 and we should't set up interrupts for each gpio chip in
that case.

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