drm/panel: tpo-td028ttec1: Fix SPI alias

The panel-tpo-td028ttec1 driver incorrectly includes the OF vendor
prefix in its SPI alias. Fix it.

Fixes: 415b8dd0

 ("drm/panel: Add driver for the Toppoly TD028TTEC1 panel")
Reported-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191007170801.27647-5-laurent.pinchart@ideasonboard.com
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Tested-by: H. Nikolaus Schaller <hns@goldelico.com>
Tested-by: Andreas Kemnade <andreas@kemnade.info>