r8169: use helper rtl_is_8168evl_up for setting register MaxTxPacketSize

>From RTL8168e-vl the value in register MaxTxPacketSize is interpreted
differently, therefore use new helper rtl_is_8168evl_up to set this
register.

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