staging: comedi: ni_mio_common: use cfc_handle_events()

The ni_event() function in this file is an open coded version of
cfc_handle_events() where instead of calling the subdevice (*cancel)
function a switch is used to call the function based on the subdevice
'index'.

Use the comedi_fc helper function to simplify the code.

This also allows removing a couple of the forward declarations.

Fix the #ifdef/#endif for the ni_gpct_cancel() foward declaration and
around the function. This function is now only referenced by code that
is blocked with #ifdef PCIDMA.

This source file is included by the ni_atmio, ni_mio_cs, and ni_pcimio
drivers. In the Kconfig, the config options for these drivers already
select COMEDI_FC.

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>