usb: r8a66597: add locking to r8a66597_check_detect_child

Use mutex usb_bus_idr_lock to protect idr_find.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>