ath10k: don't call quiet mode if it's not implemented

qca6174 and wmi-tlv doesn't have quiet mode
implemented. Don't even attempt to call it. This
fixes a warning when bringing first interface up:

  failed to set quiet mode period 100 duarion 0 enabled 0 ret -95

Fixes: 8515b5c7 ("ath10k: configure thermal throttle while powering up")
Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>