Staging: comedi: pcl816: Check channel list in AI command test

Check the channel list is valid in step 5 of the AI command test.
Split function check_and_setup_channel_list() in two.  Also, remove
unnecessary chanlist_len tests in step 3 of the AI command test as the
comedi core has already checked it.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>