mana: Initialize ports with the common RSS key

[kbowling]

Summary

Use rss_getkey() for each port's initial key instead of a lazily generated
driver-global random key.  Remove the obsolete helper and its unsynchronized
initialization flag.  Make the common-key declaration available without
options RSS.

The port still retains the key used for hardware programming and the
existing SIOCGIFRSSKEY response.  Leave hash types and queue placement
unchanged.

MFC after:      2 weeks
Sponsored by:   BBOX.io