gpio: max77620: Replace 8 with MAX77620_GPIO_NR

The MAX77620_GPIO_NR enum value represents the total number of GPIOs,
let's use it instead of a raw value in order to improve the code's
readability a tad.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Laxman Dewangan <ldewangan@nvidia.com>
Link: https://lore.kernel.org/r/20200709171203.12950-2-digetx@gmail.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>