net: mdio-gpio: Use gpio subsystem to handle low-active pins

gpiod functions support handling low-active pins, so we can move
thos code out of this driver into the gpio subsystem and simplify
the code a bit.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>