xhci: remove unused code

Value from xfer->packet.ep is assigned to ep here, but that
stored value is not used before it is overwritten. Remove it.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>