r8169: improve rtl8169_set_features

__rtl8169_set_features is used in rtl8169_set_features only, so we
can inline it. In addition:
- Remove check (features ^ dev->features), __netdev_update_features
  check's already that requested features differ from current ones.
- Don't mask out unsupported flags, there's no benefit in it.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>