mmc: core: Avoid returning error when clok scaling devfreq is removed

When resume clock scaling is not possible because there is no devfreq
framework, it is not an error. So avoid printing an error and return
success. This is necessary in certain scenarios like during shutdown
where we unregister the clock scaling devfreq.

Change-Id: Ib37eabdcdbbb58a336ee87220e5758bb559625af
Signed-off-by: Vijay Viswanath <vviswana@codeaurora.org>