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>

src/flash/nor/tcl.c

@@ -1239,12 +1239,14 @@ static const struct command_registration flash_config_command_handlers[] = {
 		.mode = COMMAND_CONFIG,
 		.handler = handle_flash_init_command,
 		.help = "Initialize flash devices.",
+		.usage = "",
 	},
 	{
 		.name = "banks",
 		.mode = COMMAND_ANY,
 		.handler = handle_flash_banks_command,
 		.help = "Display table with information about flash banks.",
+		.usage = "",
 	},
 	{
 		.name = "list",