drivers: char: msm_smd_pkt: Return error in case driver is not ready

In some cases RPMSG callback is called before RPMSG probe complete.

Return error code from call back to avoid invalid access when probe
is not completed.

Change-Id: Ib3dbec47e44d7542c8d3d44aefe3cb750384f5b5
Signed-off-by: Arun Kumar Neelakantam <aneela@codeaurora.org>