usb: xhci: Make usb_host_endpoint.hcpriv survive endpoint_disable()

xHCI hardware maintains its endpoint state between add_endpoint()
and drop_endpoint() calls followed by successful check_bandwidth().
So does the driver.

Core may call endpoint_disable() during xHCI endpoint life, so don't
clear host_ep->hcpriv then, because this breaks endpoint_reset().

If a driver calls usb_set_interface(), submits URBs which make host
sequence state non-zero and calls usb_clear_halt(), the device clears
its sequence state but xhci_endpoint_reset() bails out. The next URB
malfunctions: USB2 loses one packet, USB3 gets Transaction Error or
may not complete at all on some (buggy?) HCs from ASMedia and AMD.
This is triggered by uvcvideo on bulk video devices.

The code was copied from ehci_endpoint_disable() but it isn't needed
here - hcpriv should only be NULL on emulated root hub endpoints.
It might prevent resetting and inadvertently enabling a disabled and
dropped endpoint, but core shouldn't try to reset dropped endpoints.

Document xhci requirements regarding hcpriv. They are currently met.

Fixes: 18b74067ac78 ("xhci: Fix use-after-free regression in xhci clear hub TT implementation")
Cc: stable@vger.kernel.org
Signed-off-by: Michal Pecio <michal.pecio@gmail.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://patch.msgid.link/20260402131342.2628648-26-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>