igb: remove IGB_DESC_UNUSED since it is better handled by a function call

This patch removes IGB_DESC_UNUSED and replaces it with a function call
instead in order to cleanup some of the ugliness introduced by the macro.

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