netfilter: nf_tables: disallow NFTA_SET_ELEM_KEY_END with NFT_SET_ELEM_INTERVAL_END flag

These are mutually exclusive, actually NFTA_SET_ELEM_KEY_END replaces
the flag notation.

Fixes: 7b225d0b ("netfilter: nf_tables: add NFTA_SET_ELEM_KEY_END attribute")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>