fel: Improve file_upload() when no progress callback is passed

Until now, the function would always pass `true` as the "progress"
parameter to aw_write_buffer(). This has the potential drawback
of limiting the maximum USB transfer size.

By selectively passing `false` instead (with no progress function
active), we hint that aw_write_buffer() and subsequent routines
don't have to care about callbacks; so that usb_bulk_send() is
free to select a transfer size of AW_USB_MAX_BULK_SEND.
Reviewed-by: Bernhard Nortmann <bernhard.nortmann@web.de>

fel.c

@@ -1492,7 +1492,7 @@ static unsigned int file_upload(libusb_device_handle *handle, size_t count,
 		void *buf = load_file(argv[i * 2 + 1], &size);
 		if (size > 0) {
 			uint32_t offset = strtoul(argv[i * 2], NULL, 0);
-			aw_write_buffer(handle, buf, offset, size, true);
+			aw_write_buffer(handle, buf, offset, size, progress != NULL);
 
 			/* If we transferred a script, try to inform U-Boot about its address. */
 			if (get_image_type(buf, size) == IH_TYPE_SCRIPT)