USB: opticon: fix return value of tiocmset

Make sure we return 0 or a negative error number appropriate for
userspace on errors.

Currently 1 rather than 0 is returned on successful operation.

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