net: dsa: add slave to master helper

Many part of the DSA slave code require to get the master device
assigned to a slave device. Remove dsa_master_netdev() in favor of a
dsa_slave_to_master() helper which does that.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>