mdio-mux-gpio: Use GPIO descriptor interface and new gpiod_set_array function

Convert mdio-mux-gpio to the GPIO descriptor interface and use the new
gpiod_set_array function to set all output signals simultaneously.

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