netfilter: nft_set_pipapo_avx2: fix up description warnings

netfilter: nft_set_pipapo_avx2: fix up description warnings

W=1:
net/netfilter/nft_set_pipapo_avx2.c:159: warning: Excess function parameter 'len' description in 'nft_pipapo_avx2_refill'
net/netfilter/nft_set_pipapo_avx2.c:1124: warning: Function parameter or member 'key' not described in 'nft_pipapo_avx2_lookup'
net/netfilter/nft_set_pipapo_avx2.c:1124: warning: Excess function parameter 'elem' description in 'nft_pipapo_avx2_lookup'

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>