net: ena: changes to RSS hash key allocation

This commit contains 2 cosmetic changes:

1. Use ena_com_check_supported_feature_id() in
   ena_com_hash_key_fill_default_key() instead of rewriting
   its implementation. This also saves us a superfluous admin
   command by using the cached value.

2. Change if conditions in ena_com_rss_init() to be clearer.

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>