WQ is currently using vnic_wq_buf to store mbuf pointers for Tx
packets. But, it contains an unused mempool pointer and mbuf is
unnecessarily cast to void pointer. Remove vnic_wq_buf entirely and
use an mbuf pointer array instead.
Signed-off-by: Hyong Youb Kim <hyonkim@cisco.com> Reviewed-by: John Daley <johndale@cisco.com>