cmd: misc: Fix return value for the sleep command

If sleeping has been interrupted, return CMD_RET_FAILURE instead of -1
(CMD_RET_USAGE).

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>