ath10k: use station's current operating mode from assoc request

The current number of spatial streams used by the client is advertised
as a separate IE in assoc request. Use this information to set
the NSS operating mode.

Fixes: 45c9abc0

 ("ath10k: implement more versatile set_bitrate_mask").
Signed-off-by: Vivek Natarajan <nataraja@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>