mmc: sh_mmcif: Simplify calculation of mmc->f_min

There is no need to tune mmc->f_min to a value near 400kHz as the MMC core
begins testing frequencies at 400kHz regardless of the value of mmc->f_min.

As suggested by Guennadi Liakhovetski.

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