i2c: mvtwsi: Streamline code and add documentation

i2c: mvtwsi: Streamline code and add documentation

Convert groups of logically connected preprocessor defines into proper
enums, one macro into an inline function, and add documentation
to/extend existing documentation of these items.

Signed-off-by: Mario Six <[email protected]>
Reviewed-by: Stefan Roese <[email protected]>