Check whether the phy-reset GPIO is valid, prior to requesting it.

In the case a board does not provide a phy-reset GPIO, just returns immediately.

With such gpio validation in place, it is also safe to change from pr_debug to
dev_err in the case the gpio request fails.
Signed-off-by: NFabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>