[][MAC80211][mt76][Fix ibf cal phase issue for mt7915D]

[Description]
Fix ibf cal phase issue for mt7915D (2*2 case)
The phase calculation should be based on nss rather than tx_antenna_mask

[Release-log]
N/A

Change-Id: Ifd1589821b54074c219bf7e657c49d17e5c33006
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/7838368
2 files changed