net: ena: fix incorrect default RSS key

Bug description:
When running "ethtool -x <if_name>" the key shows up as all zeros.

When we use "ethtool -X <if_name> hfunc toeplitz hkey <some:random:key>" to
set the key and then try to retrieve it using "ethtool -x <if_name>" then
we return the correct key because we return the one we saved.

Bug cause:
We don't fetch the key from the device but instead return the key
that we have saved internally which is by default set to zero upon
allocation.

Fix:
This commit fixes the issue by initializing the key to a random value
using netdev_rss_key_fill().

Fixes: 1738cd3e

 ("net: ena: Add a driver for Amazon Elastic Network Adapters (ENA)")
Signed-off-by: Sameeh Jubran <sameehj@amazon.com>
Signed-off-by: Arthur Kiyanovski <akiyano@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>