staging: comedi: ni_tio: tidy up ni_tio_set_clock_src() and helpers

Do some renaming of local vars, etc. to tidy up the ugly line breaks
to improve the readability of the code.

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>