mt76: fix A-MPDU density handling

The hardware requirements for A-MPDU density are entirely on the tx side,
not the rx side. Because of that, the IE value should stay at 0 and the
minimum value should instead be enforced in WTBL/TXWI

MT7615 has no restrictions here

Signed-off-by: Felix Fietkau <nbd@nbd.name>