stm32 fsdev dcd: set ISO IN count inside the critical section

A VALID single-buffered ISO IN starts sending as soon as its count is set. Setting the count before entering the section let a host IN token send the packet before iso_in_sending was armed, so the USB ISR ignored the completion.

src/portable/st/stm32_fsdev/dcd_stm32_fsdev.c

diff --git a/src/portable/st/stm32_fsdev/dcd_stm32_fsdev.c b/src/portable/st/stm32_fsdev/dcd_stm32_fsdev.c
index 1ca945f..bd0c299 100644
--- a/src/portable/st/stm32_fsdev/dcd_stm32_fsdev.c
+++ b/src/portable/st/stm32_fsdev/dcd_stm32_fsdev.c
@@ -719,10 +719,10 @@
   }
   xfer->queued_len += len;
 
-  btable_set_count(ep_ix, buf_id, len);
-
   // Re-read inside the section: the fifo copy above may take the fifo mutex, so it stays outside
   usbd_critical_enter(in_isr);
+  // A VALID single-buffered ISO IN sends as soon as the count is set, so set it with iso_in_sending
+  btable_set_count(ep_ix, buf_id, len);
   // Set inside the section so a spurious ISO CTR cannot claim the packet before it is armed
   if (is_iso) {
     xfer->iso_in_sending = true;