mt76: mt7615: introduce mt7615_wtbl_desc data structure

Generalize mt7615_rate_desc introducing mt7615_wtbl_desc and
mt7615_key_desc data structures in order to configure the hw wtbl
in a non-atomic context for usb devices

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