net: dsa: Do not suspend/resume closed slave_dev

[ Upstream commit a94c689e ]

If a DSA slave network device was previously disabled, there is no need
to suspend or resume it.

Fixes: 24462549 ("net: dsa: allow switch drivers to implement suspend/resume hooks")
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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>