cxl/region: uninitialized variable in alloc_hpa()

This should check "p->res" instead of "res" (which is uninitialized).

Fixes: 23a22cd1

 ("cxl/region: Allocate HPA capacity to regions")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/Yueor88I/DkVSOtL@kili


Signed-off-by: Dan Williams <dan.j.williams@intel.com>