ethtool: Document behavior when module EEPROM bank attribute is omitted

The kernel assumes bank 0 when 'ETHTOOL_MSG_MODULE_EEPROM_GET' is sent
without 'ETHTOOL_A_MODULE_EEPROM_BANK'.

Document it as part of the interface documentation.

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