iwlwifi: fix double free/complete in firmware loading

Linus reported that due to mac80211 failing to register
the device (due to WoWLAN) his machine crashed etc. as
we double-freed the vmalloc() firmware area. His patch
to fix it was very similar to this one but I noticed
that there's another bug in the area: we complete the
completion before starting, so since we're running in
a work struct context stop() could be called while in
the middle of start() which will almost certainly lead
to issues.

Make a modification similar to his to avoid the double-
free but also move the completion to another spot so it
is only done after start() either finished or failed so
that stop() can have a consistent state.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>