net: dsa: make unexported dsa_link_touch() static

dsa_link_touch() is not exported, or defined outside of the
file it is in so make it static to avoid the following warning:

net/dsa/dsa2.c:127:17: warning: symbol 'dsa_link_touch' was not declared. Should it be static?

Signed-off-by: Ben Dooks (Codethink) <ben.dooks@codethink.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>