mt76: move mt76 rate definitions in mt76x02-lib module

Move mt76x2_rate definition in mt76x02-lib module and rename it in
mt76x02_rates in order to be reused in mt76x0 driver.
Moreover remove unused mt76_rate definition

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>