staging: comedi: adv_pci1710: tidy up pci171x_ai_check_chanlist()

Tidy up this function to clarify what the chanlist is being checked for.

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>