ACPI, PCI, irq: remove redundant check for null string pointer

source is decleared as a 4 byte char array in struct acpi_pci_routing_table
so !prt->source is a redundant null string pointer check. Detected with
smatch:

drivers/acpi/pci_irq.c:134 do_prt_fixups() warn: this array is probably
  non-NULL. 'prt->source'

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>