net: ena: Fix all static chekers' warnings

After running Sparse checker on the driver using
    make C=1 M=drivers/net/ethernet/amazon/ena

the only error that is thrown is:
    sparse: sparse: Using plain integer as NULL pointer
about the line
    struct ena_calc_queue_size_ctx calc_queue_ctx = { 0 };

This patch fixes this warning, thus making our driver free (for now) of
Sparse errors/warnings.

To make a more complete work, this patch also fixes all static warnings
that were found using an internal static checker.

Signed-off-by: Ido Segev <idose@amazon.com>
Signed-off-by: Shay Agroskin <shayagr@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>