docs: i2c: smbus-protocol: fix kernel-doc function syntax

This clarifies these are functions (and would/will adds a hyperlink to the
function documentation if/when documented).

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>