xhci: Enable LPM support only for hardwired or BESL devices

Some usb3 devices falsely claim they support usb2 hardware Link PM
when connected to a usb2 port. We only trust hardwired devices
or devices with the later BESL LPM support to be LPM enabled as default.

[Note: Sarah re-worked the original patch to move the code into the USB
core, and updated it to check whether the USB device supports BESL,
instead of checking if the xHCI port it's connected to supports BESL
encoding.]

This patch should be backported to kernels as old as 3.11, that
contain the commit a558ccdc "usb: xhci:
add USB2 Link power management BESL support".  Without this fix, some
USB 3.0 devices will not enumerate or work properly under USB 2.0 ports
on Haswell-ULT systems.

Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: stable@vger.kernel.org