bcm63xx integrated ethernet mac supports receiving and sending frames
bigger than 1500 bytes, this patch adds support for changing MTU.
This patch also fixes the reception of 802.1q frames for default MTU
which were reported as oversized.
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@16302 3c298f89-4303-0410-b956-a3cf2f4a3e73