net: ena: re-organize code to improve readability

Restructure some ethtool to a switch-case blocks to make it more uniform
with other similar functions.
Also restructure variable declaration to create reversed x-mas tree.

Signed-off-by: Arthur Kiyanovski <akiyano@amazon.com>
Signed-off-by: Shay Agroskin <shayagr@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>