mac80211: fix CSA beacon allocation size

If the FTM responder settings are changed simultaneously with
the CSA beacon, the buffer size allocated isn't sufficient and
we'll have a heap overrun. Fix this.

While at it, also clean up the ftm_responder assignment, doing
it only if ftm_responder is non-zero is valid as it's 0 to start
with, but not really useful to understand the code.

Fixes: bc847970 ("mac80211: support FTM responder configuration/statistics")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>