net: ethtool: Export helpers for getting EEPROM info

There are two ways to retrieve information from SFP EEPROMs.  Many
devices make use of the common code, and assign the sfp_bus pointer in
the netdev to point to the bus holding the SFP device. Some MAC
drivers directly implement ops in there ethool structure.

Export within net/ethtool the two helpers used to call these methods,
so that they can also be used in the new netlink code.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>