net/bonding: Move slave state changes to a helper function

Move slave state changes to a helper function, this is a pre-step for adding
functionality of dispatching an event when this helper is called.

This commit doesn't add new functionality.

Signed-off-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>