USB Endpoint struct is no longer Send

[mattico]

Previously at least <embassy_stm32::usb::Driver<'d, USB_OTG_HS> as Driver<'d>>::EndpointIn was Send, but after #2881 it contains an UnsafeCell<*mut u8> and therefore isn't Send. Perhaps we just need an unsafe impl Send for Endpoint<_> but I haven't thought carefully about whether that's safe or not.

[Dirbaio]

maybe unsafe impl Send/Sync for EpState is a less heavy hammer and should still do it. the previous code already had that for State.