ath10k: Fix the wrong updation of SGI in tx_stats debugfs

The SGI is updated wrongly in tx stats table in debugfs per sta
entry. To know whether the packets/bytes are sent with SHORT GI,
test whether the SGI bit(ATH10K_RATE_INFO_FLAGS_SGI_BIT) is set or
not in the txrate flags.

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>