net: Make GRETH driver depend on SPARC.

Reported by Stephen Rothwell.
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/Kconfig

@@ -997,7 +997,7 @@ config ETHOC
 
 config GRETH
 	tristate "Aeroflex Gaisler GRETH Ethernet MAC support"
-	depends on OF
+	depends on SPARC
 	select PHYLIB
 	select CRC32
 	help