iio: adc: mcp3422: write bit operations using bitfield.h APIs

iio: adc: mcp3422: write bit operations using bitfield.h APIs

Replace manual bit manipulations with FIELD_GET(), FIELD_PREP() and
FIELD_MODIFY() calls. The resulting code is more readable and
maintainable, and 6 macros previously defined in the header are not
needed anymore.

Signed-off-by: Marcelo Machado Lage <marcelomlage@usp.br>
Co-developed-by: Vinicius Lira <vinilira@usp.br>
Signed-off-by: Vinicius Lira <vinilira@usp.br>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>