thunderbolt: Do not send UNSET_INBOUND_SBTX when retimer NVM authentication started

Once retimer NVM authentication is started, sending UNSET_INBOUND_SBTX
will fail so avoid doing that. Only send it when we are writing an image
with not authentication or when the authentication failed early.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>