nfp: fix RSS hash key size when RSS is not supported

nfp: fix RSS hash key size when RSS is not supported

The nfp_net_get_rxfh_key_size() function returns -EOPNOTSUPP when
devices don't support RSS, and callers treat the negative value as a
large positive value since the return type is u32.

Return 0 when devices don't support RSS, aligning with the ethtool
interface .get_rxfh_key_size() that requires returning 0 in such cases.

Fixes: 9ff304bfaf58 ("nfp: add support for reporting CRC32 hash function")
Signed-off-by: Kohei Enju <enjuk@amazon.com>
Link: https://patch.msgid.link/20250929054230.68120-1-enjuk@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>