i2c: tegra: Add missing kerneldoc for some fields

Not all fields were properly documented. Add kerneldoc for the missing
fields to prevent the build from flagging them.

Reported-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>