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>