net: dsa: move definitions from dsa_priv.h to slave.c

There are some definitions in dsa_priv.h which are only used from
slave.c. So move them to slave.c.

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>