cmd: clk: Handle ENODEV from clk_get_rate

clk_get_rate may return -ENODEV if the clock isn't valid.

Also, make the error cases go through a single path.

Fixes: ff8eee03 ("cmd: clk: Add trivial implementation of clock dump
	for DM")

Signed-off-by: Ismael Luceno <ismael.luceno@silicon-gears.com>
Reviewed-by: Matthias Brugger <mbrugger@suse.com>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>