mt76: mt7915: improve MU stability

- Adjust starec flow since VHT MU group is only updated by starec_vht
  follows starec_bf settings.
- Drop unnecessary MU BF checks.

TX MPDU PER (Status = Success):
                                           TOT_MPDU_CNT  FAIL_MPDU_CNT  TX_PER
WCID Rate
1      VHT_BW80_2SS_MCS7_LGI_LDPC_MUBF              114              0   0.00%
       VHT_BW80_2SS_MCS7_LGI_LDPC_MUBF_MU            64              0   0.00%
       VHT_BW80_2SS_MCS7_SGI_LDPC_MUBF              128              0   0.00%
       VHT_BW80_2SS_MCS7_SGI_LDPC_MUBF_MU           745              0   0.00%
       VHT_BW80_2SS_MCS8_LGI_LDPC_MUBF_MU           856              0   0.00%
       VHT_BW80_2SS_MCS8_SGI_LDPC_MUBF_MU          1430              4   0.28%
       VHT_BW80_2SS_MCS9_LGI_LDPC_MUBF_MU          5220             31   0.59%
       VHT_BW80_2SS_MCS9_LGI_LDPC_iBF                59              0   0.00%
       VHT_BW80_2SS_MCS9_SGI_LDPC_MUBF               64              2   3.12%
       VHT_BW80_2SS_MCS9_SGI_LDPC_MUBF_MU         22132             76   0.34%
       VHT_BW80_2SS_MCS9_SGI_LDPC_iBF              2866              1   0.03%
2      VHT_BW80_2SS_MCS7_LGI_LDPC_MUBF_MU          3781              5   0.13%
       VHT_BW80_2SS_MCS7_SGI_LDPC_MUBF_MU           735              0   0.00%
       VHT_BW80_2SS_MCS8_LGI_LDPC_MUBF_MU          1270            365  28.74%
       VHT_BW80_2SS_MCS8_SGI_LDPC_MUBF_MU          3420             57   1.67%
       VHT_BW80_2SS_MCS9_LGI_LDPC_MUBF              128              0   0.00%
       VHT_BW80_2SS_MCS9_LGI_LDPC_MUBF_MU            64              0   0.00%
       VHT_BW80_2SS_MCS9_SGI_LDPC_MUBF              191              0   0.00%
       VHT_BW80_2SS_MCS9_SGI_LDPC_MUBF_MU         18833            320   1.70%
       VHT_BW80_2SS_MCS9_SGI_LDPC_iBF              6040              0   0.00%
287    OFDM 6M

Tested-by: Evelyn Tsai <evelyn.tsai@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>