net: ena: ethtool: support set_channels callback

Set channels callback enables the user to change the count of queues
used by the driver using ethtool. We decided to currently support only
equal number of rx and tx queues, this might change in the future.

Also rename dev_up to dev_was_up in ena_update_queue_count() to make
it clearer.

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