mac80211: fix error path for TDLS setup

The patch "8f02e6b mac80211: make sure TDLS peer STA exists during
setup" broke TDLS error paths where the STA doesn't exist when sending
the error.
Fix it by only testing for STA existence during a non-error flow.

Signed-off-by: Arik Nemtsov <arikx.nemtsov@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>