drivers: base: use device get/put functions

Use the [get|put]_device functions for ref'ing the memory block device
rather than the kobject functions which should be hidden away by the
device layer.

Signed-off-by: Seth Jennings <sjenning@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>