ar71xx: allow mtu > 1500 based on the configured tx/rx mtu register value

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37748 3c298f89-4303-0410-b956-a3cf2f4a3e73