ACPI/PCI: Remove useless NULL pointer checks

Local variable 'p' is initialized by an address of field of acpi_resource,
so it does not make sense to compare 'p' with NULL.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Signed-off-by: Andrey Strachuk <strochuk@ispras.ru>
Reviewed-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>