Set the avalon preferred packet size to 512.

driver-avalon.c

@@ -594,6 +594,7 @@ static bool avalon_detect_one(libusb_device *dev, struct usb_find_devices *found
 	/* Even though this is an FTDI type chip, we want to do the parsing
 	 * all ourselves so set it to std usb type */
 	avalon->usbdev->usb_type = USB_TYPE_STD;
+	avalon->usbdev->PrefPacketSize = AVALON_USB_PACKETSIZE;
 
 	/* We have a real Avalon! */
 	avalon_initialise(avalon);

driver-avalon.h

@@ -32,6 +32,7 @@
 #define AVALON_DEFAULT_ASIC_NUM 0xA
 
 #define AVALON_FTDI_READSIZE 510
+#define AVALON_USB_PACKETSIZE 512
 #define AVALON_READBUF_SIZE 8192
 #define AVALON_RESET_TIMEOUT 100
 #define AVALON_READ_TIMEOUT 18 /* Enough to only half fill the buffer */