qtnfmac: allow to control DFS slave radar detection

In ETSI region DFS slave device can operate in two modes on DFS channels:
 - do on-channel radar detection and use higher Tx power
 - don't do radar detection and use lower Tx power as a consequence

Allow user to control that behavior through qtnfmac module parameter.

Signed-off-by: Igor Mitsyanko <igor.mitsyanko.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>