nl80211: Simplify error handling path in 'nl80211_trigger_scan()'

Re-write the end of 'nl80211_trigger_scan()' with a more standard, easy to
understand and future proof version.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/20200712173551.274448-1-christophe.jaillet@wanadoo.fr


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