bna: Code Cleanup and Enhancements

Change details:
 -      Remove unnecessary prefetch
 -      Simplify checking & comparison of CQ flags
 -      Dereference & store unmap_array, unmap_cons & current unmap_array
        element only once
 -      Make structures tx_config & rx_config cache line aligned.

Signed-off-by: Rasesh Mody <rmody@brocade.com>
Signed-off-by: David S. Miller <davem@davemloft.net>