net: ena: allow setting the hash function without changing the key

Current code does not allow setting the hash function without
changing the key. This commit enables it.

To achieve this we separate ena_com_get_hash_function() to 2 functions:
ena_com_get_hash_function() - which gets only the hash function, and
ena_com_get_hash_key() - which gets only the hash key.

Also return 0 instead of rc at the end of ena_get_rxfh() since all
previous operations succeeded.

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>