target/adi_v5_swd: add "usage" field to command "swd"

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

Change-Id: I5a36c0fc822c4d5d74ef2d1c75e369648c6f7d4d
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4830
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>

src/target/adi_v5_swd.c

@@ -352,6 +352,7 @@ static const struct command_registration swd_handlers[] = {
 		.mode = COMMAND_ANY,
 		.help = "SWD command group",
 		.chain = swd_commands,
+		.usage = "",
 	},
 	COMMAND_REGISTRATION_DONE
 };