Update the i2c documentation: kzalloc should be used instead of
kmalloc.

I also fixed a couple other things nearby in writing-clients, as several
past changes had never been reported there.
Signed-off-by: NJean Delvare <khali@linux-fr.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>