mt76: mt7915: introduce mt7915_mcu_set_txbf()

Use mt7915_mcu_set_txbf() to reduce global functions. This can be
easily extended to support other TxBF commands in further patches.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>