cfg80211/mac80211: fix FTM settings across CSA

When FTM is enabled, doing a CSA will unexpectedly lose it since
the value of ftm_responder may be initialized to 0 instead of -1,
so fix that.

Fixes: 81e54d08 ("cfg80211: support FTM responder configuration/statistics")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>