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

Documentation/networking/ethtool-netlink.rst

@@ -1383,6 +1383,8 @@ Request contents:
   ``ETHTOOL_A_MODULE_EEPROM_I2C_ADDRESS``  u8      page I2C address
   =======================================  ======  ==========================
 
+If ``ETHTOOL_A_MODULE_EEPROM_BANK`` is not specified, bank 0 is assumed.
+
 Kernel response contents:
 
  +---------------------------------------------+--------+---------------------+