i2c: sh_mobile: optimize irq entry

We can simply pass the pointer to the private structure to the irq
routine instead of passing the platform device and looking up its
driver_data.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>