net: dsa: Bring back device detaching in dsa_slave_suspend()

Commit 448b4482 ("net: dsa: Add lockdep class to tx queues to avoid
lockdep splat") removed the netif_device_detach() call done in
dsa_slave_suspend() which is necessary, and paired with a corresponding
netif_device_attach(), bring it back.

Fixes: 448b4482 ("net: dsa: Add lockdep class to tx queues to avoid lockdep splat")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>