wifi: mt76: mt7921: fix reporting of TX AGGR histogram

Similar to mt7915, fix stats clash between bins [4-7] in 802.11 tx
aggregation histogram.

Fixes: 163f4d22 ("mt76: mt7921: add MAC support")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>