net: pppoe - get rid of DECLARE_MAC_BUF

While was playing with PPP namespaces I occasionally brought
back DECLARE_MAC_BUF which is not needed (we have %pM here).
Fix it.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>