cxgb4: Add support to enable logging of firmware mailbox commands

Add new /sys/kernel/debug/ support to dump a firmware mailbox command
issued and replies for debugging purpose.

Based on original work by Casey Leedom <leedom@chelsio.com>

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