ppp: slight optimization of addr compare

Use possibly more efficient ether_addr_equal
to instead of memcmp.

Cc: Michal Ostrowski <mostrows@earthlink.net>
Cc: netdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ding Tianhong <dingtianhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>