comedi: amplc_dio200_common: Refactor register access functions

comedi: amplc_dio200_common: Refactor register access functions

The `dio200_read8()`, `dio200_write8()`, `dio200_read32()` and
`dio200_write32()` functions apply a right-shift to the register offset
for some devices and then perform the actual register access.  Factor
the register access part out to new functions `dio200___read8()`,
`dio200___write8()`, `dio200___read32()`, and `dio200___write32()`.
This will reduce duplicated code in a subsequent patch that will
conditionally compile support for port I/O as part of the `HAS_IOPORT`
changes.

Cc: Arnd Bergmann <arnd@kernel.org>
Cc: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/20230913170712.111719-12-abbotti@mev.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>