mt76: move mt76x2u_remove_dma_hdr in mt76x02-lib module

Move mt76x2u_remove_dma_hdr in mt76x02-lib module and rename it in
mt76x02_remove_dma_hdr. Moreover use mt76x02_remove_hdr_pad routine
in mt76x02_remove_dma_hdr function.
Furthermore remove mt76x0_tx_skb_remove_dma_overhead routine

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>