dsa: fix promiscuity leak on slave dev open error

DSA master netdev promiscuity counter was not being properly
decremented on slave device open error path.

Signed-off-by: Gilad Ben-Yossef <gilad@benyossef.com>
CC: Gilad Ben-Yossef <giladb@ezchip.com>
CC: David S. Miller <davem@davemloft.net>
CC: Florian Fainelli <f.fainelli@gmail.com>
CC: Guenter Roeck <linux@roeck-us.net>
CC: Andrew Lunn <andrew@lunn.ch>
CC: Scott Feldman <sfeldma@gmail.com>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>