thermal: imx_scu_thermal: prevent boot hang with zero pdata

Should initialization of pdata values have failed for some reason,
we end up in endless loop when getting the CPU temperature value
and can not boot. Check alert value in pdata and only retry reading
temperature if alert value is not zero. Also shorten the temperature
info string.

Signed-off-by: Anatolij Gustschin <agust@denx.de>