i2c: rcar: refactor private flags

Use BIT macro to avoid shift-31-problem, indent a little more and use
GENMASK to make it easier to add new flags.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Ulrich Hecht <uli+renesas@fpond.eu>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>