mmc: core: Update HS400 timing mode before performing tuning

There is no need to prepare anything additional before tuning for HS400
other than updating the HS400 timing mode, so just do that.

Change-Id: Ic3d2a1a90b7f2701a37816caad9de4d224a6a4b0
Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>
Signed-off-by: Ram Prakash Gupta <rampraka@codeaurora.org>