drm/dp_mst: Fix drm_dp_send_dpcd_write() return code

drm_dp_mst_wait_tx_reply() returns > 1 if time elapsed in
wait_event_timeout() before check_txmsg_state(mgr, txmsg) evaluated to
true. However, we make the mistake of returning this time from
drm_dp_send_dpcd_write() on success instead of returning the number of
bytes written - causing spontaneous failures during link probing:

[drm:drm_dp_send_link_address [drm_kms_helper]] *ERROR* GUID check on
10:01 failed: 3975

Yikes! So, fix this by returning the number of bytes written on success
instead.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Fixes: cb897542

 ("drm/dp_mst: Fix W=1 warnings")
Cc: Benjamin Gaignard <benjamin.gaignard@st.com>
Cc: Sean Paul <sean@poorly.run>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Sean Paul <sean@poorly.run>
Link: https://patchwork.freedesktop.org/patch/msgid/20200424190722.775284-1-lyude@redhat.com