usb: Push USB2 LPM disable on disconnect into USB core.

The USB core currently handles enabling and disabling optional USB power
management features during device transitions (device suspend/resume,
driver bind/unbind, device reset, and device disconnect).  Those
optional power features include Latency Tolerance Messaging (LTM),
USB 3.0 Link PM, and USB 2.0 Link PM.

The USB core currently enables LPM on device enumeration and disables
USB 2.0 Link PM when the device is reset.  However, the xHCI driver
disables LPM when the device is disconnected and the device context is
freed.  Push the call up into the USB core, in order to be consistent
with the core handling all power management enabling and disabling.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>