i2c: rcar: optimize cacheline to minimize HW race condition

'flags' and 'io' are needed first, so they should be at the beginning of
the private struct.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Wolfram Sang <wsa@kernel.org>