mac80211: replace backticks by $(...)

This replaces deprecated backticks by more versatile $(...) syntax.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

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

@@ -1040,7 +1040,7 @@ list_phy_interfaces() {
 drv_mac80211_teardown() {
 	wireless_process_kill_all
 
-	for phy in `ls /sys/class/ieee80211/`; do
+	for phy in $(ls /sys/class/ieee80211/); do
 		mac80211_interface_cleanup "$phy"
 		uci -q -P /var/state revert wireless._${phy}
 	done

package/kernel/mac80211/scripts/import-backports.sh

@@ -70,7 +70,7 @@ get_next() {
 	done | tail -n1
 }
 
-CUR=`get_next`
+CUR=$(get_next)
 CUR="${CUR:-$BASE}"
 
 while [ -n "$1" ]; do