xhci: Avoid dead ports when CONFIG_USB_XHCI_HCD=n

If the user chooses to say "no" to CONFIG_USB_XHCI_HCD on a system
with an Intel Panther Point chipset, the PCI quirks code or the EHCI
driver will switch the ports over to the xHCI host, but the xHCI driver
will never load.  The ports will be powered off and seem "dead" to the
user.

Fix this by only switching the ports over if CONFIG_USB_XHCI_HCD is
either compiled in, or compiled as a module.

This patch should be backported to stable kernels as old as 3.0,
that contain commit 69e848c2


"Intel xhci: Support EHCI/xHCI port switching."

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Reported-by: Eric Anholt <eric.anholt@intel.com>
Reported-by: David Bein <d.bein@f5.com>
Cc: stable@vger.kernel.org