iio: adc: mcp3911: fix device dependent mappings for conversion result registers

The conversion result registers differs between devices. Make sure the
mapping is correct by using a device dependent .get_raw() callback function.

Fixes: 732ad342 ("iio: adc: mcp3911: add support for the whole MCP39xx family")
Co-developed-by: Lukas Rauber <lukas.rauber@janitza.de>
Signed-off-by: Lukas Rauber <lukas.rauber@janitza.de>
Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/20250428-mcp3911-fixes-v2-1-406e39330c3d@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>