Convert cmd_usage() calls in common to use a return value

Change all files in common/ to use CMD_RET_USAGE instead of calling
cmd_usage() directly. I'm not completely sure about this patch since
the code since impact is small (100 byte or so on ARM) and it might
need splitting into smaller patches. But for now here it is.

Signed-off-by: Simon Glass <sjg@chromium.org>