greybus: operation: fix use-after-free in response receive path

Fix potential use-after-free in response receive path, due to lack of
reference counting when looking up operations on a connection.

Make sure to acquire a reference to the operation while holding the
connection-list lock.

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