yang: Limit eigrp to just 1 instance per vrf

yang: Limit eigrp to just 1 instance per vrf

Currently EIGRP has built in yang code that expects only
1 ASN used per vrf.  Let's just limit the operator from
putting themselves in a bad position by allowing something like
this:

router eigrp 33
....
!
router eigrp 99
...
!

no router eigrp 99 would crash because of assumptions
made in the yang processing.

Let's just hard code that assumption into the EIGRP yang
at the moment such that it will not allow you to enter
a `router eigrp 99` instance at all.

This is purely a software limitation to prevent the code
from violating it's current assumptions.  I do not see
much need to support this at this point in time so I
fixed the problem this way instead of having to possibly
touch a bunch of code.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>