mt76: mt7921: properly configure rcpi adding a sta to the fw

Properly configure rcpi based on association process rssi. rcpi is used
by rate controller embedded into the fw to initialize amsdu size.

Tested-by: Jayden.Kuo <jayden.kuo@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>