soc: qcom: smd_debug: Add EPROBE_DEFER check to debug_ch()

debug_ch() accesses SMEM to parse the SMD structures.  If SMEM is not
initialized, an EPROBE_DEFER pointer is returned, which is incorrectly
dereferenced thus causing a crash.

Add an explicit check for this scenario to prevent the crash from
occurring.

Change-Id: Ie8ca21c29b5b8073c0f1198c68e37d3be471a93a
Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>