mmc: core: Fix updating clock scaling statistics

Due to stop transmission mechanism to handle high priority requests,
the update of clock scaling statistics is broken as the stop command
will re-enter host through mmc_start_request() and modifies ->start.busy
timestamp. If statistics are updated after sending stop command then
the duration of the interrupted command will be left unaccounted.

CRs-Fixed: 473385
Change-Id: Id671388efbfdd16981a262c0b1e5b95d119f51ae
Signed-off-by: Konstantin Dorfman <kdorfman@codeaurora.org>
Signed-off-by: Sujit Reddy Thumma <sthumma@codeaurora.org>