mt76: mt7915: fix sparse warning cast from restricted __le16

drivers/net/wireless/mediatek/mt76/mt7915/init.c:339:26: sparse:
sparse: cast from restricted __le16

Fixes: 00b2e16e

 ("mt76: mt7915: add TxBF capabilities")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>