mt76: mt7915: fix incorrect testmode ipg on band 1 caused by wmm_idx

Fix the issue that the measured inter packet gap didn't fit its
setting value.

Fixes: c2d3b192 ("mt76: mt7915: add support for ipg in testmode")
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>