ixgbe: cleanup IXGBE_DESC_UNUSED

This patch just replaces the IXGBE_DESC_UNUSED macro with a like named
inline function ixgbevf_desc_unused. The inline function makes the logic
a bit more readable.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>