xhci: Export symbols used by host-controller drivers

In preparation for allowing the xHCI host controller drivers to be built
as separate modules, export symbols from the xHCI core that may be used
by the host controller drivers.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>