net: mdio: rename mdio_device reset to reset_gpio

This renames the GPIO reset of mdio devices from 'reset' to
'reset_gpio' to better differentiate between GPIO and
reset-controller driven reset line.

Signed-off-by: David Bauer <mail@david-bauer.net>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>