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 <[email protected]>
Fixes: cb89754 ("drm/dp_mst: Fix W=1 warnings")
Cc: Benjamin Gaignard <[email protected]>
Cc: Sean Paul <[email protected]>
Acked-by: Alex Deucher <[email protected]>
Reviewed-by: Sean Paul <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

Author: Lyude

drivers/gpu/drm/drm_dp_mst_topology.c

@@ -3442,8 +3442,12 @@ static int drm_dp_send_dpcd_write(struct drm_dp_mst_topology_mgr *mgr,
 	drm_dp_queue_down_tx(mgr, txmsg);
 
 	ret = drm_dp_mst_wait_tx_reply(mstb, txmsg);
-	if (ret > 0 && txmsg->reply.reply_type == DP_SIDEBAND_REPLY_NAK)
-		ret = -EIO;
+	if (ret > 0) {
+		if (txmsg->reply.reply_type == DP_SIDEBAND_REPLY_NAK)
+			ret = -EIO;
+		else
+			ret = size;
+	}
 
 	kfree(txmsg);
 fail_put: