mt76: mt7615: rework wtbl key configuration

Remove key dependency from mt7615_mac_wtbl_update_key and export
mt7615_mac_wtbl_update_key, mt7615_mac_wtbl_update_pk and
mt7615_mac_wtbl_update_cipher in order to reuse them in usb code.
Move mt7615_mac_get_cipher in mac.h

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