net: dsa: change state setter scope

The scope of the functions inside net/dsa/slave.c must be the slave
net_device pointer. Change to state setter helper accordingly to
simplify callers.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>