Input: synaptics-rmi4 - fix kerneldoc warnings

Fixes the kerneldoc warnings from building the driver with W=1.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20201104162427.2984742-2-lee.jones@linaro.org
[dtor: folded together several Lee's patches; added more descriptions]
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>