staging: comedi: ni_tio: validate clock source

Refactor the functions that determine the clock source bits so that they
return -EINVAL if the clock source is invalid. Pass the errno back to
ni_tio_insn_config().

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>