描述一下这个bug / Describe the bug

使用USB CV第9章测试，遇到这种接口 描述符 bAlternateSetting
---------------- Interface Descriptor -----------------
bLength : 0x09 (9 bytes)
bDescriptorType : 0x04 (Interface Descriptor)
bInterfaceNumber : 0x02 (Interface 2)
bAlternateSetting : 0x00
bNumEndpoints : 0x00 (Default Control Pipe only)
bInterfaceClass : 0x01 (Audio)
bInterfaceSubClass : 0x02 (Audio Streaming)
bInterfaceProtocol : 0x00
iInterface : 0x00 (No String Descriptor)
Data (HexDump) : 09 04 02 00 00 01 02 00 00 .........

    ---------------- Interface Descriptor -----------------

bLength : 0x09 (9 bytes)
bDescriptorType : 0x04 (Interface Descriptor)
bInterfaceNumber : 0x02 (Interface 2)
bAlternateSetting : 0x01
bNumEndpoints : 0x01 (1 Endpoint)
bInterfaceClass : 0x01 (Audio)
bInterfaceSubClass : 0x02 (Audio Streaming)
bInterfaceProtocol : 0x00
iInterface : 0x00 (No String Descriptor)
Data (HexDump) : 09 04 02 01 01 01 02 00 00 .........

会报以下错误：
FAIL (USB: 1.2.51) A successful GetInterface request must return the alternate setting set by a prior call to SetInterface.

原因，usbd_core.c USB_REQUEST_GET_INTERFACE简单的赋值为0 导致的，需要USB_REQUEST_SET_INTERFACE备份alt，再返回。

复现步骤 / To Reproduce

必现

设备信息 / Target Device

STM32F407

日志 / Log

FAIL (USB: 1.2.51) A successful GetInterface request must return the alternate setting set by a prior call to SetInterface.

配置 / Configuration

No response

USB中断 / USB Interrupt

No response

缓存 / Cache

No response

商业 / Business

• 是 / Yes