ixgbe: fix compilation for VF

The mailbox API changed in version 15 and these array ops were left out
of the recent code import as applicable to FreeBSD.

Reported by: vishwin, yasu
Differential Revision: https://reviews.freebsd.org/D46708

(cherry picked from commit 267f223f622fdbf779cc43b5a564bb02e2b59ecd)