netfilter: nft_set_hash: skip fixed hash if timeout is specified

Fixed hash supports to timeouts, so skip it. Otherwise, userspace hits
EOPNOTSUPP.

Fixes: 6c03ae21 ("netfilter: nft_set_hash: add non-resizable hashtable implementation")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>