ena: Add TX IPv6 csum offload to driver_supported_features

Add a new driver supported feature bit for TX IPv6 checksum offload.
Device will expose TX IPv6 csum offload support only for drivers with
this supported feature bit on, this is in order to expose it for new
drivers only.

Approved by: cperciva (mentor)
MFC after: 2 weeks
Sponsored by: Amazon, Inc.