ath10k: fix invalid NSS for 4x4 devices

The number of spatial streams that are derived from chain mask
for 4x4 devices is using wrong bitmask and conditional check.
This is affecting downlink throughput for QCA99x0 devices. Earlier
cfg_tx_chainmask is not filled by default until user configured it
and so get_nss_from_chainmask never be called. This issue is exposed
by recent commit 166de3f1 ("ath10k: remove supported chain mask").
By default maximum supported chain mask is filled in cfg_tx_chainmask.

Cc: stable@vger.kernel.org
Fixes: 5572a95b ("ath10k: apply chainmask settings to vdev on creation")
Signed-off-by: Rajkumar Manoharan <rmanohar@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>