net/mlx5: Fix atomic_mode enum values

The field atomic_mode is 4 bits wide and therefore can hold values
from 0x0 to 0xf. Remove the unnecessary 20 bit shift that made the values
be incorrect. While that, remove unused enum values.

Fixes: 57cda166 ("net/mlx5: Add DCT command interface")
Signed-off-by: Moni Shoua <monis@mellanox.com>
Reviewed-by: Artemy Kovalyov <artemyko@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>