thunderbolt: Return -ENOTCONN when ERR_CONN is received

commit 463e48fa upstream.

This allows the calling code to distinguish if the error was due to
ERR_CONN (adapter is disconneced or disabled) or something else. Will be
needed in USB4 router NVM update in the following patch.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Liwei Song <liwei.song@windriver.com>