drm/dp_mst: Fix GUID DPCD write to non-root MST branch devices

The return value on success of drm_dp_send_dpcd_write() called for
non-root MST branch devices from drm_dp_check_mstb_guid() is the number
of bytes transferred. Atm this return value (in case of a complete read)
will be regarded incorrectly as an error by the caller of
drm_dp_check_mstb_guid(). Fix this by converting the return value for a
complete read to the expected success code (0) and for a partial read to
a failure code (-EPROTO).

Fixes: 2554da0de3e8 ("drm/display: dp-mst-topology: use new DCPD access helpers")
Cc: Dmitry Baryshkov <[email protected]>
Cc: Lyude Paul <[email protected]>
Reviewed-by: Jani Nikula <[email protected]>
Reviewed-by: Lyude Paul <[email protected]>
Signed-off-by: Imre Deak <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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