bonding: set sysfs device_type to 'bond'

Sets the sysfs device_type to 'bond' for udev. This allows udev rules to
be created for bond devices. This is similar to how other network
devices set their device_type.

Signed-off-by: Doug Goldstein <cardoe@cardoe.com>
Signed-off-by: David S. Miller <davem@davemloft.net>