USB: serial: iuu_phoenix: fix led-activity helpers

The set-led command is eight bytes long and starts with a command byte
followed by six bytes of RGB data and ends with a byte encoding a
frequency (see iuu_led() and iuu_rgbf_fill_buffer()).

The led activity helpers had a few long-standing bugs which corrupted
the command packets by inserting a second command byte and thereby
offsetting the RGB data and dropping the frequency in non-xmas mode.

In xmas mode, a related off-by-one error left the frequency field
uninitialised.

Fixes: 60a8fc01 ("USB: add iuu_phoenix driver")
Reported-by: George Spelvin <lkml@sdf.org>
Link: https://lore.kernel.org/r/20200716085056.31471-1-johan@kernel.orgReviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/iuu_phoenix.c

@@ -350,10 +350,11 @@ static void iuu_led_activity_on(struct urb *urb)
 {
 	struct usb_serial_port *port = urb->context;
 	char *buf_ptr = port->write_urb->transfer_buffer;
-	*buf_ptr++ = IUU_SET_LED;
+
 	if (xmas) {
-		get_random_bytes(buf_ptr, 6);
-		*(buf_ptr+7) = 1;
+		buf_ptr[0] = IUU_SET_LED;
+		get_random_bytes(buf_ptr + 1, 6);
+		buf_ptr[7] = 1;
 	} else {
 		iuu_rgbf_fill_buffer(buf_ptr, 255, 255, 0, 0, 0, 0, 255);
 	}
@@ -370,13 +371,14 @@ static void iuu_led_activity_off(struct urb *urb)
 {
 	struct usb_serial_port *port = urb->context;
 	char *buf_ptr = port->write_urb->transfer_buffer;
+
 	if (xmas) {
 		iuu_rxcmd(urb);
 		return;
-	} else {
-		*buf_ptr++ = IUU_SET_LED;
-		iuu_rgbf_fill_buffer(buf_ptr, 0, 0, 255, 255, 0, 0, 255);
 	}
+
+	iuu_rgbf_fill_buffer(buf_ptr, 0, 0, 255, 255, 0, 0, 255);
+
 	usb_fill_bulk_urb(port->write_urb, port->serial->dev,
 			  usb_sndbulkpipe(port->serial->dev,
 					  port->bulk_out_endpointAddress),