interface: unable to to assign tx queue to last worker thread

interface: unable to to assign tx queue to last worker thread

Using the cli "set interface tx-queue", it is not possible to assign
tx queue to the last worker thread.

The reason is that vdm->first_worker_thread_index is 1. Adding that
to clib_bitmap_last_set (bitmap) exceeds vdm->last_worker_thread_index
when the CLI specifies the last worker thread.

Also make the threads argument optional to enable user to unbind a queue
from any thread.

Type: fix

Signed-off-by: Steven Luong <sluong@cisco.com>
Change-Id: I796259c20f571289c8f5a97b9418caf452d0ab3d