mt76: mt7615: introduce mt7615_mac_update_rate_desc routine

Move register configuration out of mt7615_mac_set_rates since usb
driver can't access device register in interrupt context. Introduce
mt7615_mac_update_rate_desc routine to report rate info to
mt7615_mac_set_rates

Co-developed-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>