staging: comedi: amplc_pc236: add callback to check and clear interrupt

Add an optional callback function pointer to the board data to be called
when checking if an interrupt has occurred and to clear it if it has.

Since the callback returns `bool`, change a few other `int` values to
`bool` to match.

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