greybus: es1/2: fix use-after-free in completion callback

Reset the hcpriv field before returning the message to greybus core in
the OUT-URB completion callback.

This fixes a use-after-free bug when sending responses to incoming
requests as the final reference is then dropped when the message is
returned.

Reported-by: Michael Scott <michael.scott@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>