RRM: Use dynamically allocated buffer for beacon report

RRM: Use dynamically allocated buffer for beacon report

The maximum required size for each Beacon Report element is known in
advance: it is the size of the Beacon Report element fixed fields + the
size of the Reported Frame Body subelement.

Allocate the buffer used for constructing the Beacon Report element
dynamically with the maximum needed size, instead of using a very
large static buffer.

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