resource: Mark res given to resource_assigned() as const

resource: Mark res given to resource_assigned() as const

The caller may hold a const struct resource which will trigger an
unnecessary warning when calling resource_assigned() as it will not
modify res in any way.

Mark resource_assigned()'s struct resource *res parameter const to
avoid the compiler warning.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20251219174036.16738-15-ilpo.jarvinen@linux.intel.com