axgbe: Fix setting promisc mode

Ethernet drivers should respect IFF_PROMISC rather than IFF_PPROMISC.
The latter is for user-requested promisc mode, it implies the former
but not vice versa. Some in-kernel components such as if_bridge(4) and
bpf(4) will set promisc mode for interfaces on-demand.

While here, update the debugging message to be not confusing.

This was spotted while reviewing markj@ 's work D46524.

Test from Franco shows that the interface seems to be unconditionally
initialized to promisc mode regardless of this fix. That needs further
investigation.

Reviewed by: markj, Franco Fichtner <franco@opnsense.org>
Tested by: Franco Fichtner <franco@opnsense.org>
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D46794