mwan3: in ubus rpcd script fix shell local issue

To fix the shell local issue in the ubus mwan3 rpcd shell script, move
the switch case statment into a function.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>

net/mwan3/files/usr/libexec/rpcd/mwan3

@@ -74,7 +74,9 @@ get_mwan3_status() {
 	fi
 }
 
-case "$1" in
+main () {
+
+	case "$1" in
 		list)
 			json_init
 			json_add_object "status"
@@ -130,4 +132,7 @@ case "$1" in
 				;;
 			esac
 			;;
-esac
+	esac
+}
+
+main "$@"