netfilter: xt_ipcomp: Use ntohs to ease sparse warning

0-DAY kernel build testing backend reported:

sparse warnings: (new ones prefixed by >>)

 >> >> net/netfilter/xt_ipcomp.c:63:26: sparse: restricted __be16 degrades to integer
 >> >> net/netfilter/xt_ipcomp.c:63:26: sparse: cast to restricted __be32

Fix this by using ntohs without shifting.

Tested with: make C=1 CF=-D__CHECK_ENDIAN__

Signed-off-by: Fan Du <fan.du@windriver.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>