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>

drivers/net/ethernet/realtek/r8169_main.c

@@ -2227,6 +2227,8 @@ static enum mac_version rtl8169_get_mac_version(u16 xid, bool gmii)
 		 * the wild. Let's disable detection.
 		 * { 0x7cf, 0x540,	RTL_GIGA_MAC_VER_45 },
 		 */
+		/* Realtek calls it RTL8168M, but it's handled like RTL8168H */
+		{ 0x7cf, 0x6c0,	RTL_GIGA_MAC_VER_46 },
 
 		/* 8168G family. */
 		{ 0x7cf, 0x5c8,	RTL_GIGA_MAC_VER_44 },