netfilter: nft_extdhr: Drop pointless check of tprot_set

Pablo says, tprot_set is only there to detect if tprot was set to
IPPROTO_IP as that evaluates to zero. Therefore, code asserting a
different value in tprot does not need to check tprot_set.

Fixes: 935b7f64 ("netfilter: nft_exthdr: add TCP option matching")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>