drm: msm: dsi-staging: fix bug to return proper error code

 Fix bug where proper error code is returned when core clock enable
 is called.

Change-Id: I8726f81b4f8b8026a56e7058f427dbe9955644f0
Signed-off-by: Shashank Babu Chinta Venkata <sbchin@codeaurora.org>