staging: drm/imx: ipu-dc: signedness bug in ipu_dc_init_sync()

"map" needs to be signed for the error handling to work.  In the success
case then it holds a small non-negative value.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>