net:dsa:mv88e6xxx: use watchdog ops for 6097 chip

mv88e6097 chip requires watchdog_ops to be set.

Signed-off-by: Volodymyr Bendiuga <volodymyr.bendiuga@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>