iio: adc: mcp3911: simplify usage of spi->dev

Replace the usage of `adc->spi->dev` with `dev` to make the code prettier.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Link: https://lore.kernel.org/r/20230822192259.1125792-4-marcus.folkesson@gmail.com


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