thermal: int340x_thermal: Fix a NULL vs IS_ERR() check

The intel_soc_dts_iosf_init() function doesn't return NULL, it returns
error pointers.

Fixes: 4d0dd6c1 ("Thermal/int340x/processor_thermal: Enable auxiliary DTS for Braswell")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>