net: dsa: remove tag_protocol from dsa_switch

Having the tag protocol in dsa_switch_driver for setup time and in
dsa_switch_tree for runtime is enough. Remove dsa_switch's one.

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