drivers: usb: fastboot: Fix full-speed usb descriptor

The host will report such error message if the fastboot device work in
full-speed mode: "Duplicate descriptor for config 1 interface 0
altsetting 0, skipping"

Fastboot device ack both full and high speed interface descriptors when
work in full-speed mode, that's will cause this issue.

Fix it.

Signed-off-by: qianfan Zhao <qianfanguijin@163.com>
Reviewed-by: John Keeping <john@metanate.com>