generic: jumbo frames support for rtl8366s

generic: jumbo frames support for rtl8366s

This patch adds the option to enable jumbo frames on the rtl8366s
switch. This is almost exactly the same as the first patch at
https://dev.openwrt.org/ticket/7977 and it has been requested
in a comment there.

I have tested this and it successfully allows devices on the switch
using jumbo frames to communicate.

Signed-off-by: Ash Hughes <ashley.hughes@blueyonder.co.uk>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@28267 3c298f89-4303-0410-b956-a3cf2f4a3e73