net/mlx5: Refactor the encryption key creation

Move the common code to general functions which can be used by fast
update encryption key in later patches.

Signed-off-by: Jianbo Liu <jianbol@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>