net: dsa: move dsa slave destroy code to slave.c

Move dsa slave dedicated code from dsa_switch_destroy to a new
dsa_slave_destroy function in slave.c.
Add the netif_carrier_off and phy_disconnect calls in order to
correctly cleanup the netdev state and PHY state machine.

Signed-off-by: Frode Isaksen <fisaksen@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>