lldp: correctly increase discarded count

lldp: correctly increase discarded count

Upstream commit:
    commit 32f0deeebc9172c3f5f4a4d02aab32e6904947f6
    Date: Sat, 18 Feb 2017 20:11:47 +0100

    lldpd: correctly increase discarded count

    When a frame cannot be decoded but has been guessed, increase the
    discarded count.

    Fix https://github.com/vincentbernat/lldpd/issues/223

Fixes: be53a5c447c3 ("auto-attach: Initial support for Auto-Attach standard")
Co-authored-by: Fabrizio D'Angelo <fdangelo@redhat.com>
Signed-off-by: Fabrizio D'Angelo <fdangelo@redhat.com>
Acked-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>