u_hsuart: Fix a bug in opening the SMUX port

commit 9fdf017a (tty: n_smux: add separate close notifications)
implemented SMUX_LOCAL_CLOSED notification for clients to know
when the port is ready for opening again.  Use this notification
and defer opening until the port is ready.  If the port is opened
before this notification is arrived, the open will fail and
rmnet/dun does not work.

CRs-Fixed: 509013
Change-Id: I18bf9f08029fb98d611679d3aa65d13b088da8bc
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>