vrx518_tc: allow larger MTU to support RFC4638

vrx518_tc: allow larger MTU to support RFC4638

vrx518_tc currently sets the interface maximum MTU to the ethernet default
of 1500 bytes by default via ether_setup() called from ptm_setup().

To support 1508 byte baby jumbo frames (RFC4638) for PPPoE connections
over VDSL links as already supported by the VR9 ltq_ptm driver ([1], [2])
set the interface maximum MTU to MAX_MTU.

MAX_MTU is defined in dcdp/inc/tc_common.h to 2002 bytes and this value is
used in ptm_change_mtu() and elsewhere as the maximum MTU, however this is
short circuited by checks against the interface maximum MTU.

[1]: https://forum.openwrt.org/t/fritzbox-7530-and-rfc4638-baby-jumbo-frames/181327
[2]: https://git.openwrt.org/?p=openwrt/openwrt.git;a=commit;h=8a2a20e71e2909f84dab47e51dfda9e292a6c1ae

Signed-off-by: Andrew MacIntyre <andymac@pcug.org.au>
Link: https://github.com/openwrt/openwrt/pull/16856
Signed-off-by: Robert Marko <robimarko@gmail.com>