PM: Remove unbalanced mutex_unlock() from dpm_resume()

Remove an unnecessary unlocking of dpm_list_mtx in the error path
in drivers/base/power/main.c:dpm_suspend() .

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>