axgbe: Use the common RSS key without options RSS

Fetch the common key in the PCI iflib frontend regardless of options RSS
and make the key API declaration available in both configurations.
Retain the existing hash-field selection and queue-placement branches.
The independent ARM frontend is unchanged.

Reviewed by: gallatin
MFC after: 2 weeks
Sponsored by: BBOX.io
Differential Revision: https://reviews.freebsd.org/D59794