i2c: rcar: Remove obsolete platform data support

Since commit 4baadb9e ("ARM: shmobile: r8a7778: remove obsolete
setup code"), Renesas R-Car SoCs are only supported in generic DT-only
ARM multi-platform builds.  The driver doesn't need to use platform data
anymore, hence remove platform data configuration.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
[wsa: removed now unused ret value and cast to proper enum type]
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>