net: dsa: move notifier info to private header

The DSA notifier events and info structure definitions are not meant for
DSA drivers and users, but only used internally by the DSA core files.

Move them from the public net/dsa.h file to the private dsa_priv.h file.

Also use this opportunity to turn the events into an anonymous enum,
because we don't care about the values, and this will prevent future
conflicts when adding (and sorting) new events.

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>