nl80211: fix NL80211_ATTR_FTM_RESPONDER policy

The nested policy here should be established using the
NLA_POLICY_NESTED() macro so the length is properly
filled in.

Cc: stable@vger.kernel.org
Fixes: 81e54d08 ("cfg80211: support FTM responder configuration/statistics")
Link: https://lore.kernel.org/r/20200412004029.9d0722bb56c8.Ie690bfcc4a1a61ff8d8ca7e475d59fcaa52fb2da@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>