n_smux: smux_ctl: Userspace character device interface for SMUX

QMUXD from user-space will use this interface to communicate
control plane QMI messages between APPs and QSC modem in Fusion 4.

Change-Id: I2f51f6927a6f92b106548d22c3965575d4468d5a
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@codeaurora.org>