mlxsw: spectrum_trap: Use err variable instead of directly checking func return value

When calling mlxsw_sp_rx_listener(), use err variable instead of directly
checking func return value.

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