mt76: mt7915: improve code readability in mt7915_mcu_sta_bfer_ht

Even if it is a not real issue, add missing brackets in
mt7915_mcu_sta_bfer_ht routine in order to improve code readability

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>