ixgbevf: Drop use of eitr_low and eitr_high for hard coded values

This patch drops the use of eitr_low and eitr_high as values being stored
in the adapter structure.  Since the values have no external way to be
changed they might as well just be hard coded values and save us the space
on the adapter structure.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>