usb: xhci: add comments explaining specific interrupt behaviour

usb: xhci: add comments explaining specific interrupt behaviour

HCD does not allocate or request interrupt for the xhci driver, but HCD
does free and sync xhci interrupts in some cases. Add comment detailing
in which cases HCD will free/sync xhci interrupts.

Signed-off-by: Niklas Neronin <niklas.neronin@linux.intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20240905143300.1959279-9-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>