mlxsw: core: Fix race condition in __mlxsw_emad_transmit

Under certain conditions EMAD responses can be returned from the device
even before setting trans_active. This will cause the EMAD Rx listener
to drop the EMAD response - as there are no active transactions - and
timeouts will be generated.

Fix this by setting trans_active before transmitting the EMAD skb.

Fixes: 4ec14b76

 ("mlxsw: Add interface to access registers and process events")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>