video: mx3fb: Update comment for dmaengine_prep_slave_sg() API

Commit 16052827d98fbc13c31ebad560af4bd53e2b4dd5 ("dmaengine/dma_slave:
introduce inline wrappers") changed the code to use the new API, but forgot
to update a comment.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Jiri Kosina <trivial@kernel.org>
Cc: linux-fbdev@vger.kernel.org
--
v2:
  - New
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>