wifi: mt76: mt7996: fix bss wlan_idx when sending bss_info command

The bmc_tx_wlan_idx should be the wlan_idx of the current bss rather
than peer AP's wlan_idx, otherwise there will appear some frame
decryption problems on station mode.

Fixes: 98686cd2

 ("wifi: mt76: mt7996: add driver for MediaTek Wi-Fi 7 (802.11be) devices")
Reviewed-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>