USB: cdc-acm: fix TIOCMIWAIT

The TIOCMIWAIT implementation would return -EINVAL if any of the three
supported signals were included in the mask.

Instead of returning an error in case TIOCM_CTS is included, simply
drop the mask check completely, which is in accordance with how other
drivers implement this ioctl.

Fixes: 5a6a62bd ("cdc-acm: add TIOCMIWAIT")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
Acked-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>