Reset XMEGA NVM read command to the symbolic NO_OPERATION constant rather than a hard-coded zero.

LUFA/Drivers/USB/Core/XMEGA/USBController_XMEGA.c

@@ -72,7 +72,7 @@ void USB_Init(
 	NVM.CMD  = NVM_CMD_READ_CALIB_ROW_gc;
 	USB.CAL0 = pgm_read_byte(offsetof(NVM_PROD_SIGNATURES_t, USBCAL0));
 	USB.CAL1 = pgm_read_byte(offsetof(NVM_PROD_SIGNATURES_t, USBCAL1));
-	NVM.CMD  = 0;
+	NVM.CMD  = NVM_CMD_NO_OPERATION_gc;
 
 	/* Ugly workaround to ensure an aligned table, since __BIGGEST_ALIGNMENT__ == 1 for the 8-bit AVR-GCC toolchain */
 	USB.EPPTR = ((intptr_t)&USB_EndpointTable[1] & ~(1 << 0));