net: ena: make ethtool -l show correct max number of queues

- Update ena_ethtool:ena_get_channels() to return adapter->max_io_queues
  so that ethtool -l returns the correct maximum queue number.

- Change the name of ena_calc_io_queue_num() to
  ena_calc_max_io_queue_num() as it returns the maximum number of io
  queues and actual number of queues can be smaller if changed
  by ethtool -L which is implemented in a later commit.

- Change variable name from io_queue_num to max_num_io_queues in
  ena_calc_max_io_queue_num() and ena_probe().

- Make all types of variables that convey the number and sizeof queues
  to be u32, for consistency with the API between the driver and the
  device.

Signed-off-by: Arthur Kiyanovski <akiyano@amazon.com>
Signed-off-by: Sameeh Jubran <sameehj@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>