i2c: rk3x: add proper kerneldoc header

gcc noticed the kerneldoc was wrongly formatted. Fix it!

drivers/i2c/busses/i2c-rk3x.c:164: warning:
  Cannot understand  * @grf_offset: ...
  on line 164 - I thought it was a doc line

Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>