vxlan: fix incorrect type

The protocol is 16bit, not 32bit.

Fixes: e1e5314d ("vxlan: implement GPE")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>