Since commit 755fba11 and 8ddab8dd we can not build
USB_XHCI_NEC without USB_XHCI_PCI. Correct the Kconfig
dependency.

Fixes: 755fba11 ("usb/hcd-xhci: Move qemu-xhci device to hcd-xhci-pci.c")
Reviewed-by: Sai Pavan Boddu <sai.pavan.boddu@xilinx.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20201109135300.2592982-2-philmd@redhat.com

[ kraxel: restore "default y if PCI_DEVICES" because
          "qemu-system-ppc64 -M pseries,usb=on" needs USB_XHCI_NEC=y ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>