msm: smsm: Fix compile error when CONFIG_MSM_SMD isn't selected

Fix the return value of a function that currently returns NULL
(which is identified as a pointer) but is supposed to return
unsigned integer.

Change-Id: Ief97b3a1aa9640539756e88f78f6eed4e019e317
Signed-off-by: Jay Chokshi <jchokshi@codeaurora.org>