net: mdio: extend struct mdio_bus_stat_attr instead of using dev_ext_attribute

net: mdio: extend struct mdio_bus_stat_attr instead of using dev_ext_attribute

Currently the var member of struct dev_ext_attribute is used in a very
ugly way. Extend struct mdio_bus_stat_attr instead, what allows to
simplify the code and also slightly reduces memory footprint.

Note: Member addr is renamed to avoid a conflict in macro
      MDIO_BUS_STATS_ADDR_ATTR_DECL.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://patch.msgid.link/ce9f85d2-4f72-4b15-b868-210a8ced662d@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>