cxgb4: use backdoor access to collect dumps when firmware crashed

Fallback to backdoor register access to collect dumps if firmware
is crashed.  Fixes TID, SGE Queue Context, and MPS TCAM dump collection.

Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>
Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>