Validate PB2 USB device controller before binding

Author: Grippy98

packages/bsp/pocketbeagle2-usb-gadget/pb2-usb-gadget-ncm-acm

@@ -103,7 +103,14 @@ if [ -d /sys/module/g_cdc ] || [ -d /sys/module/g_ether ] || [ -d /sys/module/g_
 fi
 
 cleanup_configfs
-UDC=$(first_udc)
+if ! UDC=$(first_udc); then
+	echo "no USB device controller found under /sys/class/udc" >&2
+	exit 1
+fi
+if [ -z "$UDC" ]; then
+	echo "USB device controller name is empty" >&2
+	exit 1
+fi
 USB_SERIAL=$(usb_serial)
 DEV_MAC=$(derive_mac 02 pb2-dev)
 HOST_MAC=$(derive_mac 06 pb2-host)