net: dsa: better scoping of slave functions

A few DSA slave functions take a dsa_slave_priv pointer as first
argument, whereas the scope of the slave.c functions is the slave
net_device structure. Fix this and rename dsa_netpoll_send_skb to
dsa_slave_netpoll_send_skb.

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>