e1000: Restore VF interface random MAC

[kbowling]

Summary

Restore rS308429 after the iflib conversion of igb(4). This
reenables random MAC address generation when attaching
to a VF with a zeroed MAC.

PR: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=253535
Reporter: Balaev PA <[e-mail]>

Test Plan

This patch restores functionality for the submitter in the PR

[markj]

sys/dev/e1000/if_em.c

1064

How does vf_ifp get set?

[kbowling]

@markj thanks I missed a hunk of the patch, here's a corrected one