net: ena: return 0 in ena_get_rxfh_key_size() when RSS hash key is not configurable

In EC2 instances where the RSS hash key is not configurable, ethtool
shows bogus RSS hash key since ena_get_rxfh_key_size() unconditionally
returns ENA_HASH_KEY_SIZE.

Commit 6a4f7dc8 ("net: ena: rss: do not allocate key when not
supported") added proper handling for devices that don't support RSS
hash key configuration, but ena_get_rxfh_key_size() has been unchanged.

When the RSS hash key is not configurable, return 0 instead of
ENA_HASH_KEY_SIZE to clarify getting the value is not supported.

Tested on m5 instance families.

Without patch:
 # ethtool -x ens5 | grep -A 1 "RSS hash key"
 RSS hash key:
 00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00

With patch:
 # ethtool -x ens5 | grep -A 1 "RSS hash key"
 RSS hash key:
 Operation not supported

Fixes: 6a4f7dc8 ("net: ena: rss: do not allocate key when not supported")
Signed-off-by: Kohei Enju <enjuk@amazon.com>
Link: https://patch.msgid.link/20250929050247.51680-1-enjuk@amazon.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>