cxgb4: fix eeprom len when diagnostics not implemented

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>