xhci: make xhci_epid_to_usbep accept XHCIEPContext

All callsites have a XHCIEPContext pointer anyway, so we can just pass
it directly instead of fiddeling with slotid and epid.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 1474965172-30321-9-git-send-email-kraxel@redhat.com