cxgb4: Add a check for flashing FW using ethtool

Don't let T4 firmware flash on a T5 adapter and vice-versa
using ethtool

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>