hostapd: add measurement report value for beacon reports

hostapd: add measurement report value for beacon reports

Add the measurement report value to the beacon reports send via ubus. It
is possible to derive from the measurement report if a station refused to
do a beacon report and why. It is important to know why a station refuses
to do a beacon-report. In particular, we should not request a beacon
report from a station again that refused a beacon-report before.

The rejection reasons can be found by looking at the bits defined by:
- MEASUREMENT_REPORT_MODE_ACCEPT
- MEASUREMENT_REPORT_MODE_REJECT_LATE
- MEASUREMENT_REPORT_MODE_REJECT_INCAPABLE
- MEASUREMENT_REPORT_MODE_REJECT_REFUSED

Suggested-by: Ian Clowes <clowes_ian@hotmail.com>
Signed-off-by: Nick Hainke <vincent@systemli.org>