amt: fix wrong type string definition

amt message type definition starts from 1, not 0.
But type_str[] starts from 0.
So, it prints wrong type information.

Fixes: cbc21dc1 ("amt: add data plane of amt interface")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>