ena: add support for the large LLQ headers in ENA

Default LLQ (Low-latency queue) maximum header size is 96 bytes and can
be too small for some types of packets - like IPv6 packets with multiple
extension. This can be fixed, by using large LLQ headers.

If the device supports larger LLQ headers, the user can activate this
feature by setting sysctl tunable 'hw.ena.force_large_llq_header' to '1'
in the /boot/loader.conf file.

In case the device isn't supporting this feature, the default value (96B)
will be used.

Submitted by: Michal Krawczyk <mk@semihalf.com>
Obtained from: Semihalf
MFC after: 2 weeks
Sponsored by: Amazon, Inc.