mlxsw: core: Trace EMAD messages

Trace EMAD messages going down to HW and up from HW. Devlink needs to be
registered before EMAD init so the trace function can be called
with valid devlink handle.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
v1->v2:
- Use trace_devlink_hwmsg directly
Signed-off-by: David S. Miller <davem@davemloft.net>