cxgb4: Add debugfs entry to enable backdoor access

Add debugfs entry 'use_backdoor' to enable backdoor access to read sge
context. By default, we read sge context's via firmware. In case of FW
issues, one can enable backdoor access via debugfs to dump sge context
for debugging purpose.

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>