mmc: sh_mmcif: mmc->f_max should be half of the bus clock

mmc->f_max should be half of the bus clock.
And now that mmc->f_max is not equal to the bus clock the
latter should be used directly to calculate mmc->f_min.

Cc: Magnus Damm <magnus.damm@gmail.com>
Tested-by: Cao Minh Hiep <hiepcm@gmail.com>
Acked-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Chris Ball <cjb@laptop.org>