net: stmmac: dwmac-rk: Don't fail if phy regulator is absent

The devicetree binding lists the phy phy as optional. As such, the
driver should not bail out if it can't find a regulator. Instead it
should just skip the remaining regulator related code and continue
on normally.

Skip the remainder of phy_power_on() if a regulator supply isn't
available. This also gets rid of the bogus return code.

Fixes: 2e12f536 ("net: stmmac: dwmac-rk: Use standard devicetree property for phy regulator")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: David S. Miller <davem@davemloft.net>