staging: comedi: addi_apci_3120: set scan length/start after programming chanlist

The APCI-3120 documentation says that the PR/PA bits should be set after the
chanlist sequence is programmed.

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>