RRM: Remove duplicate frequencies from beacon report scan request

RRM: Remove duplicate frequencies from beacon report scan request

When setting the frequencies for beacon report request scan, it is
possible that a frequency is added twice (e.g., when the same channel
appears both in the channel field and in the AP channel report
subelement). This may cause the scan request to fail.
Make sure the frequencies array contains no duplications before
requesting the scan.

Signed-off-by: Avraham Stern <avraham.stern@intel.com>