staging: comedi: amplc_pci224: factor out the 'start pacer' code

To clarify the analog output (*do_cmd) function, factor out the
code that starts the pacer.

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