clk: do not attempt to fetch clock pointer with null device

Bail out early if device returned for the parent clock is null.
This avoids warning prints like this when doing clk dump:

  dev_get_uclass_priv: null device

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tero Kristo <kristo@kernel.org>