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/jtag/drivers/arm-jtag-ew.c

@@ -490,6 +490,7 @@ static const struct command_registration armjtagew_command_handlers[] = {
 		.handler = &armjtagew_handle_armjtagew_info_command,
 		.mode = COMMAND_EXEC,
 		.help = "query armjtagew info",
+		.usage = "",
 	},
 	COMMAND_REGISTRATION_DONE
 };

src/jtag/drivers/buspirate.c

@@ -491,6 +491,7 @@ static const struct command_registration buspirate_command_handlers[] = {
 		.handler = &buspirate_handle_adc_command,
 		.mode = COMMAND_EXEC,
 		.help = "reads voltages on adc pins",
+		.usage = "",
 	},
 	{
 		.name = "buspirate_vreg",

src/jtag/drivers/jlink.c

@@ -1772,13 +1772,15 @@ static const struct command_registration jlink_config_subcommand_handlers[] = {
 		.name = "reset",
 		.handler = &jlink_handle_config_reset_command,
 		.mode = COMMAND_EXEC,
-		.help = "undo configuration changes"
+		.help = "undo configuration changes",
+		.usage = "",
 	},
 	{
 		.name = "write",
 		.handler = &jlink_handle_config_write_command,
 		.mode = COMMAND_EXEC,
-		.help = "write configuration to the device"
+		.help = "write configuration to the device",
+		.usage = "",
 	},
 	COMMAND_REGISTRATION_DONE
 };
@@ -1820,13 +1822,15 @@ static const struct command_registration jlink_subcommand_handlers[] = {
 		.name = "freemem",
 		.handler = &jlink_handle_free_memory_command,
 		.mode = COMMAND_EXEC,
-		.help = "show free device memory"
+		.help = "show free device memory",
+		.usage = "",
 	},
 	{
 		.name = "hwstatus",
 		.handler = &jlink_handle_hwstatus_command,
 		.mode = COMMAND_EXEC,
-		.help = "show the hardware status"
+		.help = "show the hardware status",
+		.usage = "",
 	},
 	{
 		.name = "usb",

src/jtag/drivers/opendous.c

@@ -215,6 +215,7 @@ static const struct command_registration opendous_command_handlers[] = {
 		.handler = &opendous_handle_opendous_info_command,
 		.mode = COMMAND_EXEC,
 		.help = "show opendous info",
+		.usage = "",
 	},
 	{
 		.name = "opendous_hw_jtag",