ath10k: Fix the incorrect updation of NSS data in tx stats

The NSS data is updated incorrectly in the tx stats as the array
indexing starts from zero.

Fix the incorrect updation of NSS data in tx_stats by taking into
consideration the array index starting from zero.

Tested HW: WCN3990
Tested FW: WLAN.HL.3.1-00784-QCAHLSWMTPLZ-1

Fixes: a904417f ("ath10k: add extended per sta tx statistics support")
Signed-off-by: Surabhi Vishnoi <svishnoi@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>