igc: Initialize helper variables in igc_neweitr()

The justification is the same as in commit
a5b5220b1807 ("e1000: Initialize helper variables in em_newitr()").

Reviewed by: kbowling
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D50547

(cherry picked from commit d6a9f49185797c6b67e517a3d83ef63436c8d4f3)