wifi-scripts: replace legacy backtick command substitutions

Use `$(...)` instead of legacy backtick command substitutions. This
seems to be the only place in the codebase that still used backticks.

Signed-off-by: Sean Khan <[email protected]>

Author: qosmio

package/network/config/wifi-scripts/files/lib/netifd/hostapd.sh

@@ -993,7 +993,7 @@ hostapd_set_bss_options() {
 						wireless_setup_vif_failed FT_KEY_CANT_BE_DERIVED
 						return 1
 					fi
-					ft_key=`echo -n "$mobility_domain/${auth_secret:-${key}}" | md5sum | awk '{print $1}'`
+					ft_key="$(echo -n "$mobility_domain/${auth_secret:-${key}}" | md5sum | awk '{print $1}')"
 
 					set_default r0kh "ff:ff:ff:ff:ff:ff,*,$ft_key"
 					set_default r1kh "00:00:00:00:00:00,00:00:00:00:00:00,$ft_key"