net: fec: use managed function devm_gpio_request_one

Using gpio_request_one will require the probe fail-out call gpio_free,
which is missing currently.  Change to use devm_gpio_request_one to
fix the problem.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>