The gswip tag was missing in the dsa_tag_protocol_to_str() function, add it.

Fixes: 79691192 ("net: dsa: Add Lantiq / Intel GSWIP tag support")
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: David S. Miller <davem@davemloft.net>