cxgb4: fix eeprom len when diagnostics not implemented

[ Upstream commit 4ca110bf ]

Ensure diagnostics monitoring support is implemented for the SFF 8472
compliant port module and set the correct length for ethtool port
module eeprom read.

Fixes: f56ec676

 ("cxgb4: Add support for ethtool i2c dump")
Signed-off-by: Manoj Malviya <manojmalviya@chelsio.com>
Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>