cmd: ubifs: Do not show usage when command fails

Return value -1 cause U-Boot to print usage message. Return value
1 (CMD_RET_FAILURE) indicates failure. So fix return value when ubifs
command starts it execution and fails.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Dhruva Gole <d-gole@ti.com>