thermal: spear13xx: checking for NULL instead of IS_ERR()

thermal_zone_device_register() never returns NULL, on error it returns and
ERR_PTR().

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Viresh Kumar <viresh.kumar@st.com>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@st.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Len Brown <len.brown@intel.com>