staging: comedi: das800: introduce das800_ind_{write, read}()

The GAS800_GAIN register contains some bits that enable indirect
writing to additional registers on the board through iobase + 2
and indirect reading of addition registers through iobase + 7.

Introduce some helper functions to handle the indirect register
write/read.

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>