dsa: Fix Kconfig dependencies.

Based upon a report by Randy Dunlap.

DSA needs PHYLIB, but PHYLIB needs NET_ETHERNET.  So, in order
to select PHYLIB we have to make DSA depend upon NET_ETHERNET.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/dsa/Kconfig

 menuconfig NET_DSA
 	bool "Distributed Switch Architecture support"
 	default n
-	depends on EXPERIMENTAL && !S390
+	depends on EXPERIMENTAL && NET_ETHERNET && !S390
 	select PHYLIB
 	---help---
 	  This allows you to use hardware switch chips that use