cxgb4: Update T6 Buffer Group and Channel Mappings

We were using t4_get_mps_bg_map() for both t4_get_port_stats()
to determine which MPS Buffer Groups to report statistics on for a given
Port, and also for t4_sge_alloc_rxq() to provide a TP Ingress Channel
Congestion Map.  For T4/T5 these are actually the same values (because they
are ~somewhat~ related), but for T6 they should return different values
(T6 has Port 0 associated with MPS Buffer Group 0 (with MPS Buffer Group 1
silently cascading off) and Port 1 is associated with MPS Buffer Group 2
(with 3 cascading off)).

Based on the original work by Casey Leedom <leedom@chelsio.com>
Signed-off-by: Arjun Vynipadath <arjun@chelsio.com>
Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>