mmc: core: fix the clock scaling handling

Devfreq's simple ondemand governor isn't calling ->target() callback with
frequency value instead it seems to be passing 0 when it wants the client
to scale down to lowest frequency in the frequency table and passes
UINT_MAX when it wants client to scale up to maximum frequency. Fix the
->target() handler to take this into consideration.

Change-Id: I68b1c527afe398ba3ae1ed02f62a85c58f6d4244
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>