ar71xx: split packets into multiple descriptors on ar716x

This improves performance when doing concurrent rx/tx on a single
ethernet MAC, e.g. when routing between VLANs.

Fixes #13072

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42328 3c298f89-4303-0410-b956-a3cf2f4a3e73