xhci: add xhci_get_virt_ep() helper

[commit b1adc42d

 upstream]

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.

Cc: stable@vger.kernel.org
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>
Signed-off-by: Carsten Schmid <carsten_schmid@mentor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>