dsa: Hide core config options; make drivers select what they need

Commit 82167cb8c6 ('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>

net/dsa/Kconfig

@@ -1,28 +1,24 @@
+config HAVE_NET_DSA
+	def_bool y
+	depends on NETDEVICES && !S390
+
+# Drivers must select NET_DSA and the appropriate tagging format
+
 config NET_DSA
 	tristate
-	default n
-	depends on EXPERIMENTAL && NETDEVICES && !S390
+	depends on HAVE_NET_DSA
 	select PHYLIB
-	---help---
-	  This allows you to use hardware switch chips that use
-	  the Distributed Switch Architecture.
 
-
-menu "Distributed Switch Architecture support"
+if NET_DSA
 
 # tagging formats
 config NET_DSA_TAG_DSA
-	bool "Original DSA packet tagging format"
-	select NET_DSA
-	default n
+	bool
 
 config NET_DSA_TAG_EDSA
-	bool "Ethertype DSA packet tagging format"
-	select NET_DSA
-	default n
+	bool
 
 config NET_DSA_TAG_TRAILER
-	bool "Trailer DSA packet tagging format"
-	select NET_DSA
-	default n
-endmenu
+	bool
+
+endif