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

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]

Fixes: 98686cd21624 ("wifi: mt76: mt7996: add driver for MediaTek Wi-Fi 7 (802.11be) devices")
Signed-off-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Link: https://patch.msgid.link/20240816094635.2391-1-shayne.chen@mediatek.com
Signed-off-by: Felix Fietkau <nbd@nbd.name>