IB/mlx5: Use MLX5_GET to correctly get end of padding mode

MLX5_GET64 was used on end_padding_mode, which is a 2-bit field.
This is wrong as the calculated offset is incorrect. Using MLX5_GET
instead of MLX5_GET64 to fix that.

Fixes: 0fb2ed66 ('IB/mlx5: Add create and destroy functionality
                     for Raw Packet QP')
Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>