net: Make GRETH driver depend on SPARC.

Reported by Stephen Rothwell.

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/Kconfig b/drivers/net/Kconfig
index 17ff15f..46af867 100644
--- a/drivers/net/Kconfig
+++ b/drivers/net/Kconfig
@@ -997,7 +997,7 @@
 
 config GRETH
 	tristate "Aeroflex Gaisler GRETH Ethernet MAC support"
-	depends on OF
+	depends on SPARC
 	select PHYLIB
 	select CRC32
 	help