usb: Prevent dead ports when xhci is not enabled

When the xHCI driver is not available, actively switch the ports to EHCI
mode since some BIOSes leave them in xHCI mode where they would
otherwise appear dead.  This was discovered on a  Dell Optiplex 7010,
but it's possible other systems could be affected.

This should be backported to kernels as old as 3.0, that contain the
commit 69e848c2 "Intel xhci: Support
EHCI/xHCI port switching."

Signed-off-by: David Moore <david.moore@gmail.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: stable@vger.kernel.org