mt76: move tx protection routines in mt76x02-lib module

Move mt76x02_set_rts_threshold and mt76x02_mac_set_tx_protection
routines in mt76x02-lib module and remove duplicated code in mt76x0
and mt76x2 drivers. Remove pci_mac.c and mt76x0/mac.c since now are
empty files

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