staging: comedi: addi_apci_3120: move AI (*do_cmdtest) to main driver

Move apci3120_ai_cmdtest() from the included source file to the main
driver source file. Tidy up Step 4 a bit during the move.

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>