wifi: mt76: mt7996: fix rate usage of inband discovery frames

For UBPR and FILS frames, the BSS_CHANGED_BEACON flag will also be set,
which causes those frames to use the beacon rate in TX descriptors.
Adjust the statement to fix this issue.

Fixes: 98686cd2 ("wifi: mt76: mt7996: add driver for MediaTek Wi-Fi 7 (802.11be) devices")
Signed-off-by: MeiChia Chiu <meichia.chiu@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>