Commit 82167cb8 ('net: dsa/slave: Fix
compilation warnings') fixed one possible invalid configuration
(NET_DSA enabled with no trailer formats) but added others: drivers
can select NET_DSA without its dependencies being met.

It's not very useful to make either the DSA core or the tagging
formats manually selectable without a driver to use them, so:

1. Define a hidden HAVE_NET_DSA option and move the dependencies of
   NET_DSA to that.  While we're at it, drop the deprecated
   EXPERIMENTAL dependency.
2. Make NET_DSA and the drivers dependent on HAVE_NET_DSA.
3. Hide the tagging format options again.
4. Make drivers select both NET_DSA and the appropriate tagging format
   option.
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>