mmc: sh_mmcif: fix clock management

Regardless of whether the MMC bus clock is the same as the PM clock on
this specific interface, it has to be managed separately. Its proper
management should also include enabling and disabling of the clock,
whenever the interface is becoming active or going idle respectively.

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