ath10k: use configured nss instead of max nss

When re-associating a station, the nss was set back to
maximum value even if user had configured small number
of tx chains.  So, pay attention to user's config in
this case as well.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>