flash/nor/str9xpec: fix help and usage for command "part_id"

Remove the numeric parameter <bank> from help and add it to usage

Change-Id: I630ca5450955210299f54e16fe5c9e8e51b3b266
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5022
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Tested-by: jenkins

src/flash/nor/str9xpec.c

@@ -1177,9 +1177,10 @@ static const struct command_registration str9xpec_config_command_handlers[] = {
 	},
 	{
 		.name = "part_id",
+		.usage = "<bank>",
 		.handler = str9xpec_handle_part_id_command,
 		.mode = COMMAND_EXEC,
-		.help = "print part id of str9xpec flash bank <num>",
+		.help = "print part id of str9xpec flash bank",
 	},
 	COMMAND_REGISTRATION_DONE
 };