commit a56f992c upstream.

This is a very old bug, but there's nothing that prevents the
timer from running while the module is being removed when we
only do del_timer() instead of del_timer_sync().

The timer should normally not be running at this point, but
it's not clearly impossible (or we could just remove this.)
Tested-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>