cmd: cpu: refactor to ensure devices are probed and improve code style

Use uclass_first_device and uclass_next_device in order to avoid exceptions
for drivers that aren't probed when cpu ops are requested.
Improve code style and fix indentations.
Fix incorrect line break when cpu info is not available.
Remove unneeded brackets.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>