ethtool: Add ethtool operation to write to a transceiver module EEPROM

Ethtool can already retrieve information from a transceiver module
EEPROM by invoking the ethtool_ops::get_module_eeprom_by_page operation.
Add a corresponding operation that allows ethtool to write to a
transceiver module EEPROM.

The new write operation is purely an in-kernel API and is not exposed to
user space.

The purpose of this operation is not to enable arbitrary read / write
access, but to allow the kernel to write to specific addresses as part
of transceiver module firmware flashing. In the future, more
functionality can be implemented on top of these read / write
operations.

Adjust the comments of the 'ethtool_module_eeprom' structure as it is
no longer used only for read access.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Danielle Ratson <danieller@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>