mmc: core: Exit clock scaling prior to removing the card

After removing the SD/MMC 'card' from the driver model we
are cancelling the pending clock scaling work which accesses
card->dev. This could cause NULL pointer issue as the card
is already removed.

CRs-Fixed: 640344
Change-Id: I8c5ee817e3f116dedf0bf4fb51eb6b70d52467b7
Signed-off-by: Pratibhasagar V <pratibha@codeaurora.org>