thermal: armada: fix a test in probe()

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

Fixes: 3d4e5184 ("thermal: armada: convert driver to syscon register accesses")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>