mmc: core: avoid returning error value for clk-scaling

This change removes the error value (-EAGAIN) that
is returned in case of invalid state for clock scaling.
Invalid state for clock scaling doesn't mean its an error,
it merely means that at the current time we won't scale.
Devfreq will invoke scaling in next interval.

Change-Id: Idbd785da83e6ed00ede2b1b09529f7c81714ccf8
Signed-off-by: Talel Shenhar <tatias@codeaurora.org>