net: ena: Remove ena_calc_queue_size_ctx struct

This struct was used to pass data from callee function to its caller.
Its usage can be avoided.

Removing it results in less code without any damage to code readability.
Also it allows to consolidate ring size calculation into a single
function (ena_calc_io_queue_size()).

Signed-off-by: Shay Agroskin <shayagr@amazon.com>
Signed-off-by: Arthur Kiyanovski <akiyano@amazon.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>