net: dsa: unexport dsa_dev_to_net_device()

dsa.o and dsa2.o are linked into the same dsa_core.o, there is no reason
to export this symbol when its only caller is local.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>