netfilter: nft_nat: Fix endianness issue reported by sparse

This patch fixes this:

CHECK   net/netfilter/nft_nat.c
net/netfilter/nft_nat.c:50:43: warning: incorrect type in assignment (different base types)
net/netfilter/nft_nat.c:50:43:    expected restricted __be32 [addressable] [usertype] ip
net/netfilter/nft_nat.c:50:43:    got unsigned int [unsigned] [usertype] <noident>
net/netfilter/nft_nat.c:51:43: warning: incorrect type in assignment (different base types)
net/netfilter/nft_nat.c:51:43:    expected restricted __be32 [addressable] [usertype] ip
net/netfilter/nft_nat.c:51:43:    got unsigned int [unsigned] [usertype] <noident>
net/netfilter/nft_nat.c:65:37: warning: incorrect type in assignment (different base types)
net/netfilter/nft_nat.c:65:37:    expected restricted __be16 [addressable] [assigned] [usertype] all
net/netfilter/nft_nat.c:65:37:    got unsigned int [unsigned] <noident>
net/netfilter/nft_nat.c:66:37: warning: incorrect type in assignment (different base types)
net/netfilter/nft_nat.c:66:37:    expected restricted __be16 [addressable] [assigned] [usertype] all
net/netfilter/nft_nat.c:66:37:    got unsigned int [unsigned] <noident>

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>