rp2040: add barrier in usb_read_ep0_setup

Some versions of gcc, in some cases, decide that it is OK to move the
read to `INTR` to right after clearing the `SETUP_REC` bit of
`SIE_STATUS`, instead of after the `memcpy`.

The rp2040 datasheet doesn't appear to say anything about how quickly
the `SETUP_REC` bit will be cleared in `INTR`, but regardless the
compiler should not be re-ordering the read like this.

Here we force the correct ordering using a memory barrier.

Signed-off-by: Lasse Dalegaard <[email protected]>

src/rp2040/usbserial.c

@@ -96,6 +96,7 @@ usb_read_ep0_setup(void *data, uint_fast8_t max_len)
                                      | USB_BUF_CTRL_AVAIL | DPBUF_SIZE);
     usb_hw->sie_status = USB_SIE_STATUS_SETUP_REC_BITS;
     memcpy(data, (void*)usb_dpram->setup_packet, max_len);
+    barrier();
     if (usb_hw->intr & USB_INTR_SETUP_REQ_BITS) {
         // Raced with next setup packet
         usb_notify_ep0();