net: dsa: add dsa_is_normal_port helper

Introduce a dsa_is_normal_port helper to check if a given port is a
normal user port as opposed to a CPU port or DSA link.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>