i2c: Update and clean up writing-clients document

* Strip trailing white space.
* Remove out-of-date or irrelevant parts.
* Insist on the fact that command is deprecated.
* Fix spelling mistakes and typos.
* Reformat code examples and function prototypes to comply with the
  kernel coding style.

Signed-off-by: Jean Delvare <khali@linux-fr.org>