mdio-mux-gpio: use new gpiod_get_array and gpiod_put_array functions

Use the new gpiod_get_array and gpiod_put_array functions
(added to mainline in the v4.1 merge window) for obtaining and
disposing of GPIO descriptors.

Cc: David Miller <davem@davemloft.net>
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Rojhalat Ibrahim <imr@rtschenk.de>
Signed-off-by: David S. Miller <davem@davemloft.net>