USB: cdc-acm: fix failed open not being detected

Fix errors during open not being returned to userspace. Specifically,
failed control-line manipulations or control or read urb submissions
would not be detected.

Fixes: 7fb57a01

 ("USB: cdc-acm: Fix potential deadlock (lockdep
warning)")

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>