realtek: mdio: convert addr to pn

Make clear that the driver instructs the hardware via a port and
its number. To be consistent with other refactoring swap variable
addr for pn in this case.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: openwrt#22830
Signed-off-by: Robert Marko <robimarko@gmail.com>

Author: plappermaul