staging: comedi: amplc_dio200: add register shift to board info

Add `mainshift` member to `struct dio200_board` to hold the amount of
left-shift required for main register offsets.  This is 0 for all the
boards currently supported so it doesn't need initializing explicitly in
any current element of `dio200_boards[]`.  It will be non-zero for some
new boards to be supported by this driver.

Modify the register access functions `dio200_read8()` and
`dio200_write8()` to take the shift into account.

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