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