gpio: max77620: Use helper variable and clarify

Most other drivers fill out the gpio_irq_chip using a
struct gpio_irq_chip *girq helper variable for ease of
reading.

We also make a habit of explicitly assigning NULL and
zero to the parent IRQs when using ordinary IRQ handlers
in the driver, mostly for code readability and
maintenance.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Cc: Dmitry Osipenko <digetx@gmail.com>
Link: https://lore.kernel.org/r/20200716092835.69176-1-linus.walleij@linaro.org