mac80211: fix use of local variable

mac80211_get_addr is called from mac80211_generate_mac, where the local variable
initialisation id="${macidx:-0}" suggests that macidx is not always defined.
Probably, idx was supposed to be used instead of $(($macidx + 1)).

Fixes: 4d99db168c ("mac80211: try to get interface addresses from wiphy sysfs 'addresses' if no mask is set")

Signed-off-by: Leon M. George <leon@georgemail.eu>

package/kernel/mac80211/files/lib/netifd/wireless/mac80211.sh

@@ -367,7 +367,7 @@ mac80211_get_addr() {
 	local phy="$1"
 	local idx="$(($2 + 1))"
 
-	head -n $(($macidx + 1)) /sys/class/ieee80211/${phy}/addresses | tail -n1
+	head -n $idx /sys/class/ieee80211/${phy}/addresses | tail -n1
 }
 
 mac80211_generate_mac() {