mt76: move mt76x02_tx in mt76x02-lib module

Move mt76x02_tx shared routine in mt76x02-lib module and remove
duplicated code. Moreover remove mt76x0/tx.c since it is an empty
file

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