sit: fix some __be16/u16 mismatches

Fixes following sparse warnings :

net/ipv6/sit.c:1509:32: warning: incorrect type in assignment (different base types)
net/ipv6/sit.c:1509:32:    expected restricted __be16 [usertype] sport
net/ipv6/sit.c:1509:32:    got unsigned short
net/ipv6/sit.c:1514:32: warning: incorrect type in assignment (different base types)
net/ipv6/sit.c:1514:32:    expected restricted __be16 [usertype] dport
net/ipv6/sit.c:1514:32:    got unsigned short
net/ipv6/sit.c:1711:38: warning: incorrect type in argument 3 (different base types)
net/ipv6/sit.c:1711:38:    expected unsigned short [unsigned] [usertype] value
net/ipv6/sit.c:1711:38:    got restricted __be16 [usertype] sport
net/ipv6/sit.c:1713:38: warning: incorrect type in argument 3 (different base types)
net/ipv6/sit.c:1713:38:    expected unsigned short [unsigned] [usertype] value
net/ipv6/sit.c:1713:38:    got restricted __be16 [usertype] dport

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>