net: dsa: isolate legacy code

This patch moves as is the legacy DSA code from dsa.c to legacy.c,
except the few shared symbols which remain in dsa.c.

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