generic: ar8216: Enable jumbo frames on AR8327 by default

This patch enables jumbo frames on AR8327 switch by default.
I have tested it on TP-Link TL-WDR3600.

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39076 3c298f89-4303-0410-b956-a3cf2f4a3e73