net: dsa: merge dsa.c into dsa2.c

There is no longer a meaningful distinction between what goes into
dsa2.c and what goes into dsa.c. Merge the 2 into a single file.

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>