igb: remove unecessary q_vector declarations and remove itr_shift

This change removes the use of itr_shift since a mac type call can be
used just as easily to identify the only HW that needs to have the itr
shifted.

In addition it removes two unecessary declarations of a q_vector
pointer from the initialization path.

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>