em(4): honor vlanhwtag offload

The FreeBSD em driver fails to properly reset the VME flag
in the e1000 CTRL register oneg the following ifconfig command

ifconfig em1 -vlanhwtag

Tested on the e1000 device emulated by QEMU, and on a real
NIC (chip=0x10d38086).

PR: 236584
Submitted by: murat@sunnyvalley.io
Reported by: murat@sunnyvalley.io
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D25286