Add SPDX license tag to the ENA driver files

Refering to guide: https://wiki.freebsd.org/SPDX the SPDX tag should not
replace the standard license text, however it should be added over the
standard license text to make the automation easier.

Because of that, the old license was kept, but the SPDX tag was added
on top of every ENA driver file.

Submited by: Michal Krawczyk <mk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Amazon, Inc
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D27117