cfg80211: docbook: fix small formatting error

docbook (or one of its friends) gets confused with semi-colons in the
argument descriptions, causing it to think that the semi-colon is
marking a new section in the description of addr_mask in wiphy struct.

Prevent this by using hyphens instead of semi-colons in the mask
example.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>