staging: comedi: amplc_pci230: Remove forward declarations 4

Move these functions and remove their forward declarations:
pci230_ai_stop(), pci230_handle_ai().  Also move pci230_ai_cancel() for
aesthetic reasons.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>