net/mlx4: fix shifts of signed values in Tx

net/mlx4: fix shifts of signed values in Tx

This patch addresses the following issues reported by cppcheck:

 [drivers/net/mlx4/mlx4_rxtx.c:266]: (error) Shifting signed 32-bit value
     by 31 bits is undefined behaviour
 [drivers/net/mlx4/mlx4_rxtx.c:624]: (error) Shifting signed 32-bit value
     by 31 bits is undefined behaviour
 [drivers/net/mlx4/mlx4_txq.c:89]: (error) Shifting signed 32-bit value by
     31 bits is undefined behaviour
 [drivers/net/mlx4/mlx4_txq.c:91]: (error) Shifting signed 32-bit value by
     31 bits is undefined behaviour

Fixes: 78e81a9844f8 ("net/mlx4: merge Tx queue rings management")
Cc: stable@dpdk.org
Reported-by: Ferruh Yigit <ferruh.yigit@intel.com>
Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>