staging: comedi: addi_apci_3120: flip 'us_UseDma' test in apci3120_cyclic_ai()

For aesthetics, flip this test and do the DMA setup first.

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>