net: ena: rss: fix failure to get indirection table

On old hardware, getting / setting the hash function is not supported while
gettting / setting the indirection table is.

This commit enables us to still show the indirection table on older
hardwares by setting the hash function and key to NULL.

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>