omapdrm: hdmi4_cec: signedness bug in hdmi4_cec_init()

"ret" needs to be signed for the error handling to work.

Fixes: 8d7f934d ("omapdrm: hdmi4_cec: add OMAP4 HDMI CEC support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>