teach ena driver about RSS kernel option

Networking is broken if the driver configures its (virtual) hardware to
use a hash algorithm (or a key) different from the one that the network
stack (software RSS) uses. This can be seen with connections initiated
from the host. The PCB will be placed into the hash table based on the
hash value calculated by the software. The hardware-calculated hash
value in reponse packets will be different, so the PCB won't be found.

Tested with a kernel compiled with 'options RSS' on an instance with ena
driver.

Reviewed by: mw, adrian
MFC after: 2 weeks
Sponsored by: Panzura
Differential Revision: https://reviews.freebsd.org/D24733