eth: revert accidental ethash cache dir change

eth/config.go

@@ -34,6 +34,7 @@ import (
 // DefaultConfig contains default settings for use on the Ethereum main net.
 var DefaultConfig = Config{
 	SyncMode:             downloader.FastSync,
+	EthashCacheDir:       "ethash",
 	EthashCachesInMem:    2,
 	EthashCachesOnDisk:   3,
 	EthashDatasetsInMem:  1,