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>