note potential issues using ep_desc in hcd_edpt_open()

[joelpmichael]

Add a comment in hcd_edpt_open() to make new contributors aware that ep_desc is allocated from the stack when called from usbh_edpt_control_open(), so if any values from ep_desc are required (eg ep_desc->wMaxPacketSize) they must be copied.

See also #3083

[HiFiPhile]

Thank you for the note, I think it's more clear with something like HCD must maintain endpoint and transfer status locally (not a native English speaker)

[hathach]

they must be copied, there is no requirement that is long-term memory, even with normal enumeration which usbh control memory is used temporarily for holding device configuration. But they can be wiped out and reused for other control transfer.