e1000: Properly retain promisc flag

From Franco:
The iflib rewrite forced the promisc flag but it was not reported
to the system. Noticed on a stock VM that went into unsolicited
promisc mode when dhclient was started during bootup.

PR: 248869
Submitted by: Franco Fichtner <franco@opnsense.org>
Reviewed by: erj@
MFC after: 3 days