msm: msm_bus: Add checks to avoid Klocwork errors

This patch adds Null pointer checks while getting bus devices
from the kernel tree. This is required to avoid Klocwork errors.

Change-Id: I761a9d177871a42aadfaa6f855be7e9c4c781e8b
Signed-off-by: Gagan Mac <gmac@codeaurora.org>