netfilter: nft_tproxy: Fix port selector on Big Endian

On Big Endian architectures, u16 port value was extracted from the wrong
parts of u32 sreg_port, just like commit 10596608 ("netfilter:
nf_tables: fix mismatch in big-endian system") describes.

Fixes: 4ed8eb65 ("netfilter: nf_tables: Add native tproxy support")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Florian Westphal <fw@strlen.de>
Acked-by: Máté Eckl <ecklm94@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>