nl80211: Fix FTM per burst maximum value

Fix FTM per burst maximum value from 15 to 31
(The maximal bits that represents that number in the frame
is 5 hence a maximal value of 31)

Signed-off-by: Aviya Erenfeld <aviya.erenfeld@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>