drm/omap: fix return value check in dsi_bind()

Fix the retrn value check which testing the wrong variable
in dsi_bind().

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>