brcm63xx: mac80211.sh skip per-vif txpower

Today we set txpower early using wl command.

In case of set:
iw dev <ifname> set txpower fixed <value>
seems some interfaces could fail and next we will
not start wpa_supplicant.

While we already set txpower early using wl interface
skip this iw set.

Signed-off-by: Janusz Dziedzic <janusz.dziedzic@iopsys.eu>