cmd: clk: correctly handle depth for clk dump

Update depth only when clock uclass is found to have correct display
of command "clk dump".

Without this patch, the displayed depth is the binding depth for
all the uclass and that can be strange as only clock uclass nodes
are displayed.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>