r8169: add support for RTL8168M

A user reported an unknown chip version. According to the r8168 vendor
driver it's called RTL8168M, but handling is identical to RTL8168H.
So let's simply treat it as RTL8168H.

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