greybus: usb: fix hcd allocation, deregistration and deallocation

Fix allocation, deregistration and deallocation of USB HCD, and update
the hcd_priv helper functions.

The HCD private data was not allocated correctly, something which would
lead to a crash when accessed in hcd_start. The HCD was neither
reregistered or deallocated on connection tear down.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>