net: dsa: move headers exported by slave.c to slave.h

Minimize the use of the bloated dsa_priv.h by moving the prototypes
exported by slave.c to their own header file.

This is just approximate to get the code structure right. There are some
interdependencies with static inline code left in dsa_priv.h, so leave
slave.h included from there for now.

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>