CMD: random: fix return code

As of today 'random' command return 1 (CMD_RET_FAILURE) in case
of successful execution and 0 (CMD_RET_SUCCESS) in case of bad
arguments. Fix that.

NOTE: we remove printing usage information from command body
so it won't print twice.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Reviewed-by: Simon Glass <sjg@chromium.org>