mt76: unify sta_add / sta_remove

Merge mt76x0 and mt76x0 sta add/remove callback. We drop
mt76x0_mac_set_ampdu_factor() for now. Need to consider to add
it to common code, but mt76x2 don't do it so perhaps mt76x0 don't
need it as well.

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