AR934x v1.3 no longer needs the DCU backoff reduction workaround for
preventing rx overruns, but in turn needs the number of usable Tx
buffers to be reduced slightly.
Signed-off-by: NFelix Fietkau <nbd@openwrt.org>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>