ARM: imx: use PTR_ERR_OR_ZERO

replace IS_ERR/PTR_ERR

Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>