mt76: mt7915: remove set but not used variable 'msta'

Cc: linux-wireless@vger.kernel.org,
    linux-arm-kernel@lists.infradead.org,
    linux-mediatek@lists.infradead.org,
    netdev@vger.kernel.org,
    kernel-janitors@vger.kernel.org

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/wireless/mediatek/mt76/mt7915/mcu.c: In function 'mt7915_mcu_sta_txbf_type':
drivers/net/wireless/mediatek/mt76/mt7915/mcu.c:1805:21: warning:
 variable 'msta' set but not used [-Wunused-but-set-variable]

It is never used, so can be removed.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>