Rework and simplify Tx DMA mapping in ENA

Driver working in LLQ mode in some cases can send only few last segments
of the mbuf using DMA engine, and the rest of them are sent to the
device using direct PCI transaction. To map the only necessary data, two DMA
maps were used. That solution was very rough and was causing a bug - if
both maps were used (head_map and seg_map), there was a race in between
two flows on two queues and the device was receiving corrupted
data which could be further received on the other host if the Tx cksum
offload was enabled.

As it's ok to map whole mbuf and then send to the device only needed
segments, the design was simplified to use only single DMA map.

The driver version was updated to v2.1.1 as it's important bug fix.

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