netfilter: reduce sparse warnings

bridge/netfilter/ebtables.c:290:26: warning: incorrect type in assignment (different modifiers)
-> remove __pure annotation.

ipv6/netfilter/ip6t_SYNPROXY.c:240:27: warning: cast from restricted __be16
-> switch ntohs to htons and vice versa.

netfilter/core.c:391:30: warning: symbol 'nfq_ct_nat_hook' was not declared. Should it be static?
-> delete it, got removed

net/netfilter/nf_synproxy_core.c:221:48: warning: cast to restricted __be32
-> Use __be32 instead of u32.

Tested with objdiff that these changes do not affect generated code.

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