net: ena: fix incorrect usage of memory barriers

Added memory barriers where they were missing to support multiple
architectures, and removed redundant ones.

As part of removing the redundant memory barriers and improving
performance, we moved to more relaxed versions of memory barriers,
as well as to the more relaxed version of writel - writel_relaxed,
while maintaining correctness.

Signed-off-by: Netanel Belgazal <netanel@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>