qlxge: Derive both RSS keys from the common key

[kbowling]

Summary

Replace the embedded default with rss_getkey().  Preserve the ICB's
little-endian representation of each big-endian key word, then copy
the initial 16 bytes of the serialized IPv6 key into the IPv4 key.

This retains the old default-key bytes and allows a configured common
key to take effect.  Leave RSS flags and queue mapping unchanged.

MFC after:      2 weeks
Sponsored by:   BBOX.io