mfd: cros_ec: Fix and improve kerneldoc comments.

cros-ec includes inside the MFD subsystem, specially the file
cros_ec_commands.h, has been modified several times and it has grown a
lot, unfortunately, we didn't have care too much about the documentation.
This patch tries to improve the documentation and also fixes all the
issues reported by kerneldoc script.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Benson Leung <bleung@chromium.org>