i2c: sh_mobile: refactor rx isr

Remove the do_while loop which was just there to have an easy exit with
"break;" and replace it with if-else-blocks which should make the state
machine clearer.

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