TTY: msm_smd_tty: Handle tty port registration failure

If the tty port registration fails, an invalid pointer could be
dereferenced.  Handle the failure so that an invalid pointer dereference
does not occur.

Change-Id: Ia7ecca441a310004db8f2f8dcad44725870ebb33
Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>