mt76: introduce mt76_sw_queue data structure

Introduce mt76_sw_queue data structure in order to support new
chipsets (e.g. mt7615) that have a shared hardware queue for all traffic
identifiers. mt76_sw_queue will be used to track outstanding packets

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>