tty: n_smux: Add function to determine if remote side is active

For subsystem restart unit tests, we need to know when the remote system
is back up to avoid having to add fixed delays.

Add private function to allow checking to see if the remote side is
active.

Change-Id: Ide2f5e4aba01479c2dc1e8c3b4332da7da6ad0c5
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>