igb: resolve warning of unused adapter struct

If DCA is undefined then the adapter struct becomes unnecessary.  To
resolve this issue the DCA calls can simply make a call to the adapter
struct through the rx_ring adapter struct member.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Acked-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>