cfg80211: fix kernel-doc for cfg80211_beacon_data

The kernel-doc comment is formatted badly, resulting
in a warning:

  include/net/cfg80211.h:1188: warning: bad line: [...]

Fix that.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>