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>

drivers/acpi/pci_irq.c

@@ -131,9 +131,6 @@ static void do_prt_fixups(struct acpi_prt_entry *entry,
 		quirk = &prt_quirks[i];
 
 		/* All current quirks involve link devices, not GSIs */
-		if (!prt->source)
-			continue;
-
 		if (dmi_check_system(quirk->system) &&
 		    entry->id.segment == quirk->segment &&
 		    entry->id.bus == quirk->bus &&