net/mlx5: Change clock in mlx5_core_dev to mlx5_clock pointer

Change clock member in mlx5_core_dev to a pointer, so it can point to
a clock shared by multiple functions in later patch.

For now, each function has its own clock, so mdev in mlx5_clock_priv
is the back pointer to the function. Later it points to one (normally
the first one) of the multiple functions sharing the same clock.

Change mlx5_init_clock() to return error if mlx5_clock is not
allocated. Besides, a null clock is defined and used when hardware
clock is not supported. So, the clock pointer is always pointing to
something valid.

Signed-off-by: Jianbo Liu <jianbol@nvidia.com>
Reviewed-by: Carolina Jubran <cjubran@nvidia.com>
Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>