xtensa/PCI: Remove unused variable

gcc reports:

arch/xtensa/kernel/pci.c:40:32: warning:
	'pci_ctrl_tail' defined but not used

which is indeed the case.
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Message-Id: <1560694321-31380-1-git-send-email-linux@roeck-us.net>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/kernel/pci.c

@@ -37,7 +37,6 @@
  */
 
 static struct pci_controller *pci_ctrl_head;
-static struct pci_controller **pci_ctrl_tail = &pci_ctrl_head;
 
 static int pci_bus_count;