msm: msm_bus: Correct the return value from register-client

On failure, return 0, and not a non-zero value as that can
be interpreted as a valid client ID.

Change-Id: I7a5b448bc398b4a4792170147a5e5317d25f8b3c
CRs-Fixed: 486328
Signed-off-by: Gagan Mac <gmac@codeaurora.org>