USB: xhci: Allow roothub ports to be disabled.

Add the hub emulation code to allow ports on an xHCI root hub to be
disabled.  Add the code to clear the port enabled/disabled bit, and clear
the port enabled/disabled change bit.  Like EHCI, the port cannot be
enabled by setting the port enabled/disabled bit.  Instead, a port is
enabled by the host controller after a reset.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>