rtwn: add (untested) radar detection support for RTL8821AU
(disabled by default).

To enable it, dev.rtwn.%d.radar_detection tunable need to be set
to a nonzero value.

Tested with RTL8821AU, AP mode (no radar events were seen - so,
I have no idea if it is really correct / working)

Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D8903