firmware loader: fix firmware -ENOENT situations

If the requested firmware image doesn't exist, firmware->priv
should be set for the later concurrent requests, otherwise
warning and oops will be triggered inside firmware_free_data().

Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>