i2c: rcar: refactor setting up msg

Setting up a read or write message is similar enough to be done in one
function. Also, move a helper function into the new function since it is
only used here.

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>