staging: drm/imx: fix spelling error for vsync flag config

partial fix of changes from
"staging: drm/imx: Add support for VGA via TVE on i.MX53"

Have to check for vsync_pin instead of hsync_pin to set Vsync_pol.

Signed-off-by: Markus Niebel <Markus.Niebel@tqs.de>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Tested-by: Philipp Zabel <p.zabel@pengutronix.de>
Tested-by: Rogerio Pimentel <rpimentel.silva@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>