net: ena: rss: do not allocate key when not supported

Currently we allocate the key whether the device supports setting the
key or not. This commit adds a check to the allocation function and
handles the error accordingly.

Fixes: 1738cd3e

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