cxgb4: Update ethtool get_drvinfo to get regdump len

Update ethtool get_drvinfo to display regdump len and also update
firmware string version print to display N/A in case FW isn't present

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