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

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 <[email protected]>
Reviewed-by: Viresh Kumar <[email protected]>
Reviewed-by: Vincenzo Frascino <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Len Brown <[email protected]>