i2c: rcar: sanity check for minimal DMA length

Use a macro for the hardcoded value and apply a build check. If it is
not met, the driver logic will not work anymore.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>