Add Rx offsets support for the ENA driver

For the first descriptor in a chain the data may start at an offset.
It is optional feature of some devices, so the driver must ack that
it supports it.

The data pointer of the mbuf is simply shifted by the given value.

Submitted by: Maciej Bielski <mba@semihalf.com>
Submitted by: Michal Krawczyk <mk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Amazon, Inc
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D27116