xhci: add asserts to help with static code analysis

xhci: add asserts to help with static code analysis

Most callers of xhci_port_update() and xhci_wakeup() pass in a pointer
to an array entry and can never be NULL but add two defensive asserts
to protect against future changes (e.g. adding a new port speed, etc.)
adding a path through xhci_lookup_port() that could result in the
return of a NULL XHCIPort.

Signed-off-by: Liam Merwick <liam.merwick@oracle.com>
Message-id: 1549460216-25808-3-git-send-email-liam.merwick@oracle.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>