net: lantiq_xrx200: convert to build_skb

We can increase the efficiency of rx path by using buffers to receive
packets then build SKBs around them just before passing into the network
stack. In contrast, preallocating SKBs too early reduces CPU cache
efficiency.

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

	Down		Up
Before	577 Mbps	648 Mbps
After	624 Mbps	695 Mbps

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