net: dsa: Export symbols for dsa_port_vid_{add, del}

This is needed so that the newly introduced tag_8021q may access these
core DSA functions when built as a module.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>