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 <dalegaard@gmail.com>
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