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)