A recent iface lock removal in ed17320c3d
exposed issues with concurrent access on TX to drivers that are not
re-entrant.
Reverting that commit does not really solve the problem, as it would
still exist if multiple Traffic Class queues are in use.
Therefore, introduce a separate mutex for TX data path, protecting the
L2/driver from concurrent transfers from several threads.
Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>
61c392c5b1