drm/dp_mst: Fix drm_dp_check_mstb_guid() return code

We actually expect this to return a 0 on success, or negative error code
on failure. In order to do that, we check whether or not we managed to
write the whole GUID and then return 0 if so, otherwise return a
negative error code. Also, let's add an error message here so it's a
little more obvious when this fails in the middle of a link address
probe.

This should fix issues with certain MST hubs seemingly stopping for no
reason in the middle of the link address probe process.

Fixes: cb897542c6d2 ("drm/dp_mst: Fix W=1 warnings")
Cc: Benjamin Gaignard <[email protected]>
Cc: Sean Paul <[email protected]>
Cc: Hans de Goede <[email protected]>
Signed-off-by: Lyude Paul <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Alex Deucher <[email protected]>

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