ARM: shmobile: use do{ }while() on SH_CLK_SET_RATIO()

SH_CLK_SET_RATIO() will be trouble without this patch

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>