usb: host: xhci: fix compilation error for non-PCI based stacks

For non PCI-based stacks, this function call
usb_disable_xhci_ports(to_pci_dev(hcd->self.controller));
made from xhci_shutdown is not applicable.

Ideally, we wouldn't have any PCI-specific code on
a generic driver such as the xHCI stack, but it looks
like we should just stub usb_disable_xhci_ports() out
for non-PCI devices.

[ balbi@ti.com: slight improvement to commit log ]

This patch should be backported to kernels as old as 3.0, since the
commit it fixes (e95829f4 "xhci: Switch
PPT ports to EHCI on shutdown.") was marked for stable.

Signed-off-by: Moiz <Sonasath&lt;m-sonasath@ti.com>
Signed-off-by: Ruchika Kharwar <ruchika@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: stable@vger.kernel.org