xhci: add xhci_get_virt_ep() helper

In several event handlers we need to find the right endpoint
structure from slot_id and ep_index in the event.

Add a helper for this, check that slot_id and ep_index are valid.

Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20210129130044.206855-6-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>