David Vrabel noticed that the wireless usb stack likes to call
device_for_each_chile() with an empty bus.  This used to work fine, but
now oopses.  This patch fixes the oops and makes the code behave like it
used to.
Reported-by: NDavid Vrabel <david.vrabel@csr.com>
Tested-by: NDavid Vrabel <david.vrabel@csr.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>