staging: comedi: ni_labpc: tidy up labpc_ai_scan_mode()

This function checks the cmd->chanlist to determine the scan mode
used to sample the analog inputs. All possible modes are checked
so the final pr_err() and return 0 can never be reached.

Tidy up the function a bit and remove the unreachable code.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>