drm/radeon/dp: return -EIO for flags not zero case

If there are error flags in the aux transaction return
-EIO rather than -EBUSY.  -EIO restarts the whole transaction
while -EBUSY jus retries.  Fixes problematic aux transfers.

Bug:
https://bugs.freedesktop.org/show_bug.cgi?id=80684



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org