liquidio: remove/replace invalid code

Remove invalid call to dma_sync_single_for_cpu() because previous DMA
allocation was coherent--not streaming.  Remove code that references fields
in struct list_head; replace it with calls to list_empty() and
list_first_entry().  Also, add comment to clarify complicated if statement.

Signed-off-by: Rick Farrington <ricardo.farrington@cavium.com>
Signed-off-by: Felix Manlunas <felix.manlunas@cavium.com>
Signed-off-by: Derek Chickles <derek.chickles@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>