eigrpd: Ensure better `struct thread *` semantics

eigrpd: Ensure better `struct thread *` semantics

1) Do not explicitly set the thread pointer to NULL.
FRR should only ever use the appropriate THREAD_ON/THREAD_OFF
semantics.  This is espacially true for the functions we
end up calling the thread for.

2) Fix mixup of `struct eigrp_interface` and `struct eigrp`
usage of the same thread pointer.

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