ath10k: apply chainmask settings to vdev on creation

It appears it takes more than just setting the
hardware's chainmask to make things work well.  Without
this patch, a vdev would only use 1x1 rates when chainmask
was set to 0x3.

Setting the 'nss' (number of spatial streams) on the vdev
helps the firmware's rate-control algorithm work properly.

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