net: dsa: rename dsa_is_normal_port helper

This patch renames dsa_is_normal_port to dsa_is_user_port because "user"
is the correct term in the DSA terminology, not "normal".

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>