cli: run_commandf(): small fixups

cli: run_commandf(): small fixups

* vsnprintf() can truncate cmd, hence it makes no sense to launch such
command (it's broken). Moreover, it's better to signalize to the caller
about such case (for facilitating debugging or bug hunting).

* Fix kernel-doc warnings:
  include/command.h:264: info: Scanning doc for run_commandf
  include/command.h:268: warning: contents before sections
  include/command.h:271: warning: No description found for return value
                                  of 'run_commandf'

* Add printf-like format attribute to validate at compile-time the format
string against parameters's type.

* Fix compilation error in case of -Wall, -Werror, -Wextra:
error: variable ‘i’ set but not used [-Werror=unused-but-set-variable]

* Drop extra ret variable.

Signed-off-by: Evgeny Bachinin <EABachinin@sberdevices.ru>
Reviewed-by: Simon Glass <sjg@chromium.org>