mmc: core: fix downdifferential for clock scaling

"downdifferential" parameter of devfreq's simple ondemand governor supposed
to report expected difference between upthreshold and downthreshold value.
In other words, downdifferential = upthreshold - downthreshold. But
currently downdifferential is set to same as downthreshold, this change
fixes this issue.

Change-Id: Ic2e762d192f1fed8f94d4d2579d6a4b5d4c2c8b5
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>