staging: comedi: range: tidy up comedi_check_chanlist()

The only difference in the if() and else if() check of the chanlist
is the source of the range table length. Consolidate the checks to
make the function a bit more concise.

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>