staging: comedi: me4000: tidy up the chanlist checking

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

For aesthetics, rename this function so it has namespace associated with the
driver and tidy up the code.

To minimize the noise, change the 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>