ath10k: implement drv_get_tsf for ibss merging

Implement the get TSF by simply returning 0 so that IBSS
merging is happening. Otherwise, IBSS nodes that have similar
SSID naming won't merge. This is simply fooling the mac80211
that the TSF in the received beacon is higher than the local TSF.

Signed-off-by: Chun-Yeow Yeoh <yeohchunyeow@gmail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>