staging: comedi: amplc_pci230: change pci230_handle_ao_fifo() return type

Change the return type of `pci230_handle_ao_fifo()` from `int` to
`bool`.  A return value of `true` indicates the AO command is still
running.

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