igb: Move DMA Coalescing init code to separate function.

This patch moves the DMA Coalescing feature initialization code from
igb_reset to a new function and replaces it with a call to the new
function.

Signed-off-by: Carolyn Wyborny <carolyn.wyborny@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>