cxgb4: fix incorrect condition for using firmware LDST commands

Only contact firmware if it's alive _AND_ if use_bd (use backdoor
access) is not set when issuing FW_LDST_CMD.

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>