tty: serial: msm: Avoid to start tx when port suspended

No need to start uart tx when the port is suspended.
Also there is a race condition during system suspend process
and flush_to_ldisc work. When uart port suspended, it
will disable clocks and if the flush_to_ldisc work try to
do a uart start tx it will cause a noc error.

Change-Id: Ia1327837de03fc1bf827b891f8a0fc4b1bfdcfec
Signed-off-by: Maria Yu <aiquny@codeaurora.org>
Signed-off-by: Tengfei Fan <tengfeif@codeaurora.org>