r8169: improve rtl_set_rx_mode

[ Upstream commit 81cd17a4121d7dc7cad28e51251f31ff12b1de2b ]

This patch improves and simplifies rtl_set_rx_mode a little.
No functional change intended.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: efa5f1311c49 ("net: r8169: Disable multicast filter for RTL8168H and RTL8107E")
Signed-off-by: Sasha Levin <sashal@kernel.org>