Add support for ENA NETMAP partial initialization

In NETMAP mode not all queues need to be allocated to NETMAP. Some of
them could be left to the kernel. Configuration is managed by the flags
nr_mode and nr_pending_mode provided per each NETMAP kring.

ENA driver checks those flags and perform proper rings initialization.

Differential Revision: https://reviews.freebsd.org/D21937
Submitted by: Rafal Kozik <rk@semihalf.com>

Michal Krawczyk <mk@semihalf.com>

Obtained from: Semihalf
Sponsored by: Amazon, Inc.