driver core: prevent device_for_each_child from oopsing
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: David Vrabel <david.vrabel@csr.com> Tested-by: David Vrabel <david.vrabel@csr.com> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>