lacp: deleting the bond subinterface may cause lacp to lose the partner [VPP-1251]

lacp: deleting the bond subinterface may cause lacp to lose the partner [VPP-1251]

Problem:
When the bond subinterface is removed, it was observed that we lost the lacp
partner. Show hardware shows rx counter goes up, but show interface does not
for the slave interfaces.

Cause:
We reset the interface promiscuous mode when the bond subinterface is deleted.
This causes dpdk not to accept any packet. Leave the interface in promiscuous
mode fixes the problem.

Other fixes:
There are few places we use hw_if_index as if they are sw_if_index. But they
don't necessarily have the same value. As soon as a subinterface is created,
they start to diverge. The fix is to use the correct API for the hw_if_index
and sw_if_index.

Change-Id: I1e6b8bca0a4aae396d217a141271cbf968500c91
Signed-off-by: Steven <sluong@cisco.com>