You can't write it using the MCHBAR mirror, the write will just get
dropped.

This should make us BSpec-compliant, but there's no real bug I could
reproduce that is fixed by this patch.
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: NRodrigo Vivi <rodrigo.vivi@gmail.com>
Reviewed-by: NDamien Lespiau <damien.lespiau@intel.com>
[danvet: Fix spelling mistake in the comment that Damien spotted.]
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>