mt76: mt76u: reduce rx memory footprint

Reduce rx memory footprint allocating just one SG buffer since for the
moment we support just 3839B as maximal size of an A-MSDU.
Introduce different SG_MAX_SIZE definitions for TX and RX sides.
Moreover set q->buf_size to PAGE_SIZE even for SG case.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>