xtensa: fix warning comparing pointer to 0

Fix the following coccicheck warning:

./arch/xtensa/kernel/pci.c:79:17-18: WARNING comparing pointer to 0.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Message-Id: <1615360238-22508-1-git-send-email-jiapeng.chong@linux.alibaba.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>