mmc: core: Fix clock scaling's init error checking

The SD card's clock scaling initialization code fails to error
check the error pointer returned from the registration with the
devfreq framework. As a result, a possible failure to register with
the devfreq framework may not be reported in a timely manner,
resulting in the SD card clock scaling feature failure later on
when it is accessed.

Change-Id: I6995d98d4f4539aa7f5f2d73eb3aafcc96ec5cf6
Signed-off-by: Bao D. Nguyen <nguyenb@codeaurora.org>