Add additional preprocessor sanity checks for the XMEGA platform based on the USB controller input clock speed.

LUFA/Drivers/USB/Core/XMEGA/Device_XMEGA.h

@@ -92,10 +92,12 @@
 			 */
 			#define USB_DEVICE_OPT_LOWSPEED        (1 << 0)
 
-			/** Mask for the Options parameter of the \ref USB_Init() function. This indicates that the
-			 *  USB interface should be initialized in full speed (12Mb/s) mode.
-			 */
-			#define USB_DEVICE_OPT_FULLSPEED       (0 << 0)
+			#if (F_USB > 6000000)
+				/** Mask for the Options parameter of the \ref USB_Init() function. This indicates that the
+				 *  USB interface should be initialized in full speed (12Mb/s) mode.
+				 */
+				#define USB_DEVICE_OPT_FULLSPEED   (0 << 0)
+			#endif
 			//@}
 
 			#if (!defined(NO_INTERNAL_SERIAL) || defined(__DOXYGEN__))

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

@@ -105,11 +105,15 @@ void USB_Disable(void)
 
 void USB_ResetInterface(void)
 {
+	#if defined(USB_DEVICE_OPT_FULLSPEED)
 	if (USB_Options & USB_DEVICE_OPT_LOWSPEED)
 	  CLK.USBCTRL = (((F_USB / 6000000) - 1) << CLK_USBPSDIV_gp);
 	else
 	  CLK.USBCTRL = (((F_USB / 48000000) - 1) << CLK_USBPSDIV_gp);
-
+	#else
+	CLK.USBCTRL = (((F_USB / 6000000) - 1) << CLK_USBPSDIV_gp);
+	#endif
+	
 	if (USB_Options & USB_OPT_PLLCLKSRC)
 	  CLK.USBCTRL |= (CLK_USBSRC_PLL_gc   | CLK_USBSEN_bm);
 	else

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

@@ -95,7 +95,7 @@
 			#error F_USB is not defined. You must define F_USB to the frequency of the unprescaled USB controller clock in your project makefile.
 		#endif
 
-		#if (F_USB % 6000000)
+		#if ((F_USB % 6000000) || (F_USB < 6000000))
 			#error Invalid F_USB specified. F_USB must be a multiple of 6MHz for USB Low Speed operation, and a multiple of 48MHz for Full Speed operation.
 		#endif