staging: comedi: addi_apci_3120: move start_src check into apci3120_cyclic_ai()

For aesthetics, move the check of the cmd->start_src.

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>