bus_remove_device: be more careful about incomplete initialization

Prevent bus_remove_device() from crashing if dev->knode_bus has not been
initialized before it's called.

This can happen if the device_add() ended up breaking out early due to
an error, for example.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>