> (5/5) Documentation update.

Finally, updates are required to the i2c/writing-client and
i2c/porting-client documents. Remove any reference to i2c_client id and
invite porters to discard that struct member.
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>