net/fec: fec_reset_phy() does not need to always succeed

FEC can work without a phy reset on some platforms, which means not
very platform necessarily have a phy-reset gpio encoded in device tree.
Even on the platforms that have the gpio, FEC can work without
resetting phy for some cases, e.g. boot loader has done that.

So it makes more sense to have the phy-reset-gpio request failure as
a debug message rather than a warning, and get fec_reset_phy() return
void since the caller does not check the return anyway.

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