devfreq: return error code when governor start fails

If there is a failure in starting hw monitor required for
memlat governor, return the error code to devfreq framework
so that the framework  will make sure the governor is
switched back to previous governor.

Change-Id: I76705725fc8b94d5341b1b8af7c1bd2b79e7642a
Signed-off-by: Santosh Mardi <gsantosh@codeaurora.org>