Add missing attribute validation for beacon report scanning
to the netlink policy.

Fixes: 1d76250b ("nl80211: support beacon report scanning")
Signed-off-by: NJakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/20200303051058.4089398-3-kuba@kernel.orgSigned-off-by: NJohannes Berg <johannes.berg@intel.com>