imx-common: cpu: Do not print on invalid temperature

It is not very useful to have the message below on every boot
(especially when we are using early silicon):

U-Boot 2015.10-rc2-23945-g37cf215 (Sep 08 2015 - 14:12:14 -0300)

CPU:   Freescale i.MX6UL rev1.0 792 MHz (running at 396 MHz)
CPU:   Commercial temperature grade (0C to 95C)CPU:   Thermal invalid data, fuse: 0x0
 - invalid sensor device

, so turn the error message into debug level.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>