ath10k: Remove passing unused argument for ath10k_mac_tx

'ath10k_mac_tx' does not seems to use the per station table
entry pointer 'sta' (struct ieee80211_sta), hence remove passing
this unused argument

Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>