Fix out of bound access of ep array.

When processing USB_REQ_CLEAR_FEATURE, USB_REQ_SET_FEATURE, and
USB_REQ_GET_STATUS packets in dwc2_ep0_setup an out of bounds access
can occur. This is caused by the wIndex field of the usb control packet
being used as an index into an array whose size is DWC2_MAX_ENDPOINTS (4).

Signed-off-by: Alison Huffman <alisn@google.com>