staging: comedi: amplc_pc236: add callback to enable/disable interrupt

Add an optional callback function pointer to the board data to be called
when interrupts are logically enabled or disabled to update the hardware
registers.

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