mmc: core: Update the logic of controlling clk scaling through sysfs

mmc clock scaling can be disabled/enabled through sysfs.
The present logic in this path deregisters/registers with devfreq
every time. Instead of this, we can simply suspend/resume the clock
scaling when requested for disabling/enabling clock scaling.
This patch updates the mentioned logic.

With original logic, observed deadlock between devfreq registration
and cmdqd thread in low memory conditions. The updated logic fixes
this deadlock condition aswell.

Change-Id: Ifee1ffbe24b13b8f5dc1c9f0579ce9ddf4b4faf3
Signed-off-by: Veerabhadrarao Badiganti <vbadigan@codeaurora.org>