usb: common: Consider only available nodes for dr_mode

There are cases where multiple device tree nodes point to the
same phy node by means of the "phys" property, but we should
only consider those nodes that are marked as available rather
than just any node.

Fixes: 98bfb394 ("usb: of: add an api to get dr_mode by the phy node")
Cc: stable@vger.kernel.org # v4.4+
Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>