igb: Split igb_update_dca into separate Tx and Rx functions

This change makes it so that igb_update_dca is broken into two halves, one
for Rx and one for Tx.  The advantage to this is primarily readability.

In addition I am enabling relaxed ordering for reads from hardware since
this is supported on all of the igb parts.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>