net: ethernet: stmmac: dwmac-rk: fix optional phy regulator handling

The usual devm_regulator_get() call already handles "optional"
regulators by returning a valid dummy and printing a warning
that the dummy regulator should be described properly. This
code open coded the same behaviour, but masked any errors that
are not -EPROBE_DEFER and is quite noisy.

This change effectively unmasks and propagates regulators errors
not involving -ENODEV, downgrades the error print to warning level
if no regulator is specified and captures the probe defer message
for /sys/kernel/debug/devices_deferred.

Fixes: 2e12f536 ("net: stmmac: dwmac-rk: Use standard devicetree property for phy regulator")
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: David S. Miller <davem@davemloft.net>