p2p: make DiscReason bigger than byte

We decode into [1]DiscReason in a few places. That doesn't work anymore
because package rlp no longer accepts RLP lists for byte arrays.

p2p/peer_error.go

@@ -57,7 +57,7 @@ func (self *peerError) Error() string {
 	return self.message
 }
 
-type DiscReason byte
+type DiscReason uint
 
 const (
 	DiscRequested DiscReason = iota