drm/dp_mst: Fix drm_dp_check_mstb_guid() return code

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: eed1ea7b2209 ("drm/dp_mst: Fix W=1 warnings")
Cc: Benjamin Gaignard <benjamin.gaignard@st.com>
Cc: Sean Paul <sean@poorly.run>
Cc: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200306234923.547873-3-lyude@redhat.com
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>