BGP: Ignore unexpected values in ENHE capability

BGP: Ignore unexpected values in ENHE capability

Silently ignore (without sending a Notification) unexpected values
of AFI, SAFI or Nexthop AFI received in the Extended Next Hop Encoding
capability (defined in RFC 5549). While this RFC only defines certain
values as allowed, that may be changed by a future spec.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by:   Daniel Walton <dwalton@cumulusnetworks.com>
Ticket: CM-5975
Reviewed By: CCR-3947
Testing Done: test_fuzz 1.11, 1.12 and 1.13