drm/imx: imx-tve: check the value returned by regulator_set_voltage()

regulator_set_voltage() may fail, so we better check its return value
and propagate it in the case of error.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>