mmc: core: fix the issue with clock scaling in HS400 mode

Clock scaling feature would scale up/down the MMC interface clock
frequency based on the load on storage. During clock scale up in HS400
bus speed mode, SW is unncesserily doing the tuning even after HS400
switch is done. This change fixes this bug.

Change-Id: Ica39acc41b14095438a7ccf536c3c7e5792d9c41
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>