staging: comedi: pcmuio: introduce pcmuio_read()

Introduce a helper function to handle reading a 24-bit value from the
three 8-bit registers associated with a "port" or "page".

Simplify the for() loop in pcmuio_handle_asic_interrupt() that finds
which channels have triggered the interrupt.

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>