sh, mmc: Use defines when setting CE_CLK_CTRL

The 16-19th bits of CE_CLK_CTRL set the
MMC clock frequency.

Cc: Yusuke Goda <yusuke.goda.sx@renesas.com>
Cc: Magnus Damm <magnus.damm@gmail.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>