qmod DFU: Don't overwrite memory beyond end of usb_strings[]

The code in board_main_top() for QMOD blindly re-assigned some
members of the usb_strings[] array, writing to index 7 and 8.

However, that array only has those entries in the main firmware,
while in DFU that array has only 6 entries.  Depending on whatever
the linker has decided to put in the next memory location after
that array, we would overwrite that very early during boot-up.

Change-Id: I59e4e1a54e819808d5a8259a6d14f4b970a90020
Related: OS#4302

firmware/libboard/qmod/source/board_qmod.c

@@ -332,11 +332,13 @@ void board_main_top(void)
 		TRACE_INFO("Detected Quad-Modem ST12\n\r");
 	} else {
 		TRACE_INFO("Detected Quad-Modem ST34\n\r");
+#ifndef APPLICATION_dfu
 		/* make sure we use the second set of USB Strings
 		 * calling the interfaces "Modem 3" and "Modem 4" rather
 		 * than 1+2 */
 		usb_strings[7] = usb_strings[9];
 		usb_strings[8] = usb_strings[10];
+#endif
 	}
 
 	/* Obtain the circuit board version (currently just prints voltage */