tty: n_smux: Add support for broadcast logical channel ID

Add support for broadcast logical channel ID used with the dedicated
power-control commands.

Without this support, power commands sent on the broadcast channel may
get dropped which is currently only a test case issue.

Change-Id: I798ed8a315b3f864fa9afa20ef073b2ab5cab293
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>