wifi: mt76: mt7996: fix incorrect indexing of MIB FW event

Fix wrong calculation of the channel times due to incorrect
interpretation from the FW event.

Fixes: 98686cd2 ("wifi: mt76: mt7996: add driver for MediaTek Wi-Fi 7 (802.11be) devices")
Signed-off-by: Benjamin Lin <benjamin-jw.lin@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Link: https://patch.msgid.link/20250114101026.3587702-4-shayne.chen@mediatek.com


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