USB: serial: fix TIOCMIWAIT return value

Fix regression introduced by commit 143d9d96 ("USB: serial: add
tiocmiwait subdriver operation") which made the ioctl operation return
ENODEV rather than ENOIOCTLCMD when a subdriver TIOCMIWAIT
implementation is missing.

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