ath79: fix patching ath9k MAC address for MyNet WiFi Range Extender

ath79: fix patching ath9k MAC address for MyNet WiFi Range Extender

The code line patching ath9k MAC address for this device contains
a wrong number of arguments including an unset "$mac", which
looks like a typo or copy/paste mistake.

This has been introduced already in the device support commit
745dee11ac78 ("ath79: add support for WD My Net Wi-Fi Range
Extender").

This patch just removes the "$mac" argument, leaving a formally
valid line. (No on-device test has been performed.)

Cc: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>