No caller of usb_ep_get() calls it with a NULL device (previous commits
have addressed the few remaining cases which didn't explicitly check).
Replace check for 'dev == NULL' with an assert instead.

Signed-off-by: Liam Merwick <liam.merwick@oracle.com>
Message-id: 1549460216-25808-10-git-send-email-liam.merwick@oracle.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>