flash_command_get_bank_by_num: make COMMAND_HELPER

Use COMMAND_HELPER macro to declare flash_command_get_bank_by_num.
This is required for COMMAND_PARSE_NUMBER macro.

src/flash/lpc2000.c

@@ -750,7 +750,7 @@ COMMAND_HANDLER(lpc2000_handle_part_id_command)
 	}
 
 	struct flash_bank *bank;
-	int retval = flash_command_get_bank_by_num(cmd_ctx, args[0], &bank);
+	int retval = CALL_COMMAND_HANDLER(flash_command_get_bank_by_num, 0, &bank);
 	if (ERROR_OK != retval)
 		return retval;