mmc: sh_mobile_sdhi: fix clock frequency printing

During its probing the SDHI driver prints out the clock frequency, but
does it wrongly, always reporting 0Hz. Use the MMC host frequency value
to fix this issue.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Chris Ball <cjb@laptop.org>