Set empty usage field for commands that do not need parameters

Set empty usage field for commands that do not need parameters

The missing field causes runtime debug message
	BUG: command '%s' does not have the '.usage' field filled out

While there, fix some minor typo in the help messages:
	s/deasert/deassert/
	s/Deasert/Deassert/

Change-Id: If3dd18265cda103ca0d05609f67f4ca58e7cbb27
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5024
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>