MIPS: lantiq: dma: increase descritor count

NAT Performance results on BT Home Hub 5A (kernel 5.10.89, mtu 1500):

	Down		Up
Before	539 Mbps	599 Mbps
After	545 Mbps	625 Mbps

Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>