netfilter: nft_tunnel: fix sparse errors

[...]
net/netfilter/nft_tunnel.c:117:25:    expected unsigned int [unsigned] [usertype] flags
net/netfilter/nft_tunnel.c:117:25:    got restricted __be16 [usertype] <noident>
[...]
net/netfilter/nft_tunnel.c:246:33:    expected restricted __be16 [addressable] [assigned] [usertype] tp_dst
net/netfilter/nft_tunnel.c:246:33:    got int

Fixes: af308b94

 ("netfilter: nf_tables: add tunnel support")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>