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

commit f6be5e64500abbba44e191e1ca0f3366c7d0291b upstream.

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>
[bwh: Backported to 3.2: error code is returned directly here]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>