amt: fix error return code in amt_init()

Return error code when alloc_workqueue()
fails in amt_init().

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Taehee Yoo <ap420073@gmail.com>
Link: https://lore.kernel.org/r/20211102130353.1666999-1-yangyingliang@huawei.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>