Cause backpressure (instead of dropping frames) on congestion.

MFC after:	3 days

sys/dev/cxgbe/t4_main.c

@@ -492,6 +492,8 @@ t4_attach(device_t dev)
 	    V_RXTSHIFTMAXR2(15) | V_PERSHIFTBACKOFFMAX(8) | V_PERSHIFTMAX(8) |
 	    V_KEEPALIVEMAXR1(4) | V_KEEPALIVEMAXR2(9));
 	t4_write_reg(sc, A_ULP_RX_TDDP_PSZ, V_HPZ0(PAGE_SHIFT - 12));
+	t4_set_reg_field(sc, A_TP_PARA_REG3, F_TUNNELCNGDROP0 |
+	    F_TUNNELCNGDROP1 | F_TUNNELCNGDROP2 | F_TUNNELCNGDROP3, 0);
 
 	setup_memwin(sc);