ath10k: use MAC clock rate same as FW uses for survey accuracy

WLAN survey data provides several informative values on each channels
such as noise, active time, transmit time, and etc.
Regarding the channel activity time data, it is reported from firmware
in different form which is number of cycles instead of time itself.
Hence host driver converts it to time unit by dividing it by clock rate
that is used at baseband MAC.

Using difference clock rate from that actual HW is using will result in
inaccurate survey data. For instance, channel active time can be reported
with bigger than 150ms even though we set Dwell time to 150ms.
Therefore set the clock rate to 88000 (88 MHz) which is ath10k baseband
MAC is using.

Signed-off-by: Peter Oh <poh@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>