net: dsa: Add helper to convert from devlink to ds

Given a devlink instance, return the dsa switch it is associated to.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>
2 files changed