thermal: thermal_mmio: remove some dead code

The platform_get_resource() function doesn't return error pointers, it
returns NULL.  The way this is normally done, is that we pass the NULL
resource to devm_ioremap_resource() and then check for errors from that.
See the comment in front of devm_ioremap_resource() for more details.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Talel Shenhar <talel@amazon.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>