e1000: Use the common RSS key for 82574

[kbowling]

Summary

Use rss_getkey() in the older em multi-queue RSS path, which is reached
by 82574 with two receive queues.  This replaces a private key generated
on each initialization and matches the common-key policy already used
by the igb path.

Decode each register word with le32dec() and remove the private assembly
macro, avoiding signed integer shifts while preserving the register byte
order.  Leave the redirection table and hash fields unchanged.  This does
not change single-queue em devices or PF-owned igb VF RSS settings.

MFC after:      2 weeks
Sponsored by:   BBOX.io