wifi: mt76: mt7996: use hweight16 to get correct tx antenna

In the Linux kernel, the following vulnerability has been resolved: wifi: mt76: mt7996: use hweight16 to get correct tx antenna The chainmask is u16 so using hweight8 cannot get correct tx_ant. Without this patch, the tx_ant of band 2 would be -1 and lead to the following issue: BUG: KASAN: stack-out-of-bounds in mt7996_mcu_add_sta+0x12e0/0x16e0 [mt7996e]