net: dsa: introduce dsa_user_ports helper

Introduce a dsa_user_ports() helper to return the ds->enabled_port_mask
mask which is more explicit. This will also minimize diffs when touching
this internal mask.

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