staging: comedi: hwdrv_apci1500: introduce z8536_write()

The Z8536 CIO registers are indirectly written by writing the register offset
to the control register then writing the value to the control register.

Introduce a helper function to write the Z8536 CIO registers.

The registers are written from "normal" code and in the interrupt handler,
spinlock the sequence to protect the indirect register access.
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>