ar71xx: ack completed tx descriptors only after the full frame has been completed

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

Backport of r42457

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@42475 3c298f89-4303-0410-b956-a3cf2f4a3e73
Merge remote-tracking branch 'origin/master'