staging: comedi: ni_labpc: tidy up the chanlist checking

The labpc_ai_chanlist_invalid() function validates that the cmd->chanlist
is compatible with the hardware. This is step 5 of the (*do_cmdtest).

For aesthetics, rename this function and tidy up the code.

To minimize the noise, change the comedi_error(), which is a wrapper around
dev_err(), to dev_dbg().

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>