cxgb4: Use ntohs() on __be16 value instead of htons()

Use the correct direction of byte-swapping function to fix a mistake
shown by sparse endianness checking -- c.fl0id is __be16.

Signed-off-by: Roland Dreier <rolandd@cisco.com>
Acked-by: Dimitris Michailidis <dm@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>