helper/command: fix printing usage for incomplete commands

helper/command: fix printing usage for incomplete commands

The commit "helper/command: do not replace new commands with
ocd_ prefix" breaks the print of usage text when a multi-word
command is typed incompletely.

Fix it by explicitly running the "usage" command.

Change-Id: I46f8e521d9b6ac617bff8938cf8fc96f0ade45c8
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5166
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>