net: lantiq: add support for jumbo frames

Add support for jumbo frames. Full support for jumbo frames requires
changes in the DSA switch driver (lantiq_gswip.c).

Tested on BT Hone Hub 5A.

Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>