mmc: block: Disable clock scaling during shutdown

During shutdown of sdhc drivers, power off notification (PON) is sent
to eMMC device. But it doesn't prevent the clock scaling framework
from kicking in and sending commands to card after PON is sent. So
exit clock scaling framework of sdhc before PON is sent.

Change-Id: Ibfe097732042458965db529f235a7fd28cfabaab
Signed-off-by: Vijay Viswanath <vviswana@codeaurora.org>