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

@@ -74,6 +74,8 @@ get_mwan3_status() {
 	fi
 }
 
+main () {
+
 	case "$1" in
 		list)
 			json_init
@@ -131,3 +133,6 @@ case "$1" in
 			esac
 			;;
 	esac
+}
+
+main "$@"