net: dsa: mv88e6xxx: rework port mode setup

A switch port mode is defined by the association of its egress mode, its
frame mode and if supported or required, the ether type value.

Pack all this in a mv88e6xxx_set_port_mode function and provide helpers
for the Normal Network mode, the DSA mode, and the Ether Type DSA mode,
as well as an helper to setup a port's mode depending on its nature.

Define PORT_ETH_TYPE_DEFAULT for the 0x9100 reset value of port E Type.

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