amt: add missing regeneration nonce logic in request logic

When AMT gateway starts sending a new request message, it should
regenerate the nonce variable.

Fixes: cbc21dc1 ("amt: add data plane of amt interface")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>