[TCP] H-TCP: fix integer overflow

When using H-TCP with a single flow on a 500Mbit connection (or less
actually), alpha can exceed 65000, so alpha needs to be a u32.

Signed-off-by: Gavin McCullagh <gavin.mccullagh@nuim.ie>
Signed-off-by: Doug Leith <doug.leith@nuim.ie>
Signed-off-by: David S. Miller <davem@davemloft.net>