Use bitfield for storing global ENA device states

As the ENA can have multiple states turned on/off, it is more convenient
to store them in single bitfield instead of multiple boolean variables.

The bitset FreeBSD API was used for the bitfield implementation, as it
provides flexible structure together with API which also supports atomic
bitfield operations.

For better readability basic macros from API were wrapped into custom
ENA_FLAG_* macros, which are filling up common parameters for all calls.

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