realtek: mdio: rename iterator to "addr"

realtek: mdio: rename iterator to "addr"

During initialization the mdio driver registers phys with the
iterator "pn". To make clear that it is a phy address rename it
to "addr".

While we are here improve the upper bound check of the maximum
possible address. This is the family specific cpu port and not
the generic upper bound constant for all devices.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/21438
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>