staging: comedi: adv_pci1710: don't check the chanlist twice

The chanlist is checked in Step 5 of the (*do_cmdtest) there is no
reason to check it again in the (*do_cmd). The only reason its done
again is to get the actual 'seglen', the non-repeating length of the
chanlist.

Save the 'seglen' found by pci171x_ai_check_chanlist() in the private
data and use that in the (*do_cmd). Rename the private data member to
clarify it. Also, remove the unused 'act_chanlist_pos' member from the
private data.

Refactor the error handling in pci171x_ai_check_chanlist() so it returns
and errno for failure and 0 for success.

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>