sfc: Added support for new ethtool APIs for obtaining module eeprom

Currently allows for SFP+ eeprom to be returned using the ethtool API.
This can be extended in future to handle different eeprom formats
and sizes

Signed-off-by: Stuart Hodgson <smhodgson@solarflare.com>
[bwh: Drop redundant validation, comment, whitespace]
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>