mx31_3ds: Fix the logic for detecting the debug board

The function mxc_expio_init returns 0 on success, so fix the logic for
printing the error case.
Also fixed the wording on the error message.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Rogerio Pimentel <rogerio.pimentel@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>