em(4): Always reinit interface when adding/removing VLAN

This partially reverts r361053 since there have been reports
by users that this breaks some functionality for em(4)
devices; it seems at first glance that some sort of interface
restart is required for those cards.

This isn't a proper fix; this unbreaks those users until a proper
fix is found for their issues.

PR: 240818
Reported by: Marek Zarychta <zarychtam@plan-b.pwste.edu.pl>
MFC after: 3 days