mgmt: mcumgr: transport: smp_bt: Fix deadlock on disconnect with data

This fixes an issue with the bluetooth transport whereby if a device
drops the connection prior to receiving all the output data it could
cause a deadlock.

Fixes #51641

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
(cherry picked from commit 7bab5c1b)