mlxsw: pci: Add mirror reason field to CQEv2

The Completion Queue Element version 2 (CQEv2) includes a field called
'mirror_reason' which indicates why the packet was mirrored to the CPU.

Add the field so that it can be used by a later patch.

Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>