wifi: mt76: mt7996: fix endianness of MT_TXD6_TX_RATE

To avoid sparse warning:
sparse: warning: invalid assignment: |=
sparse:    left side has type restricted __le32
sparse:    right side has type unsigned lon

Fixes: 15ee62e7

 ("wifi: mt76: mt7996: enable BSS_CHANGED_BASIC_RATES support")
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/16fa938373e3b145cb07a2c98d2428fea2abadba.1682285873.git.ryder.lee@mediatek.com