wifi: mt76: fix reading current per-tid starting sequence number for aggregation

The code was accidentally shifting register values down by tid % 32 instead of
(tid * field_size) % 32.

Cc: stable@vger.kernel.org
Fixes: a28bef56 ("mt76: mt7615: re-enable offloading of sequence number assignment")
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220826182329.18155-1-nbd@nbd.name