nft-arp: remove wrong conditions

Removes wrong conditions in flags translating functions
that doesn't permit to delete rule with inverse flags set.

For instance, the following command doesn't remove the rule:
arptables-compat -D INPUT -i ! eth1 -j ACCEPT

Signed-off-by: Giuseppe Longo <giuseppelng@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>