net: dsa: Fix dsa_legacy_register() return value

We need to make the dsa_legacy_register() stub return 0 in order for
dsa_init_module() to successfully register and continue registering the
ETH_P_XDSA packet handler.

Fixes: 2a93c1a3 ("net: dsa: Allow compiling out legacy support")
Reported-by: Egil Hjelmeland <privat@egil-hjelmeland.no>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>