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

sys/dev/e1000/if_em.c

@@ -1342,7 +1342,7 @@ em_if_init(if_ctx_t ctx)
 	}
 
 	/* Don't lose promiscuous settings */
-	em_if_set_promisc(ctx, IFF_PROMISC);
+	em_if_set_promisc(ctx, if_getflags(ifp));
 	e1000_clear_hw_cntrs_base_generic(&adapter->hw);
 
 	/* MSI-X configuration for 82574 */