i2c: rcar: refactor irq state machine

Remove the seperate functions and use designated constants. As readable
but less overhead. Actually, this is even more readable since the old
function used a mix of "=" and "|=".

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>