r8169: Drop tp arg from rtl8169_tx_vlan_tag()

Since eab6d18d (vlan: Don't check for vlan group before
vlan_tx_tag_present.) we don't check tp->vlgrp and thus
tp is not needed in this function.

Signed-off-by: Kirill Smelkov <kirr@mns.spb.ru>
Acked-by: Francois Romieu <romieu@fr.zoreil.com>