ethtool: Validate module EEPROM offset as part of policy

Validate the offset to read from module EEPROM as part of the netlink
policy and remove the corresponding check from the code.

This also makes it possible to query the offset range from user space:

 $ genl ctrl policy name ethtool
 ...
 ID: 0x14  policy[32]:attr[2]: type=U32 range:[0,255]
 ...

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>