greybus: add a reference to pending operations

Grab an extra reference to an operation before sending it.  Drop
that reference at the end of its completion handling.

It turns out gb_operation_get() got deleted along the way, so this
re-introduces it.  We're assuming we only get a reference when
there's at least one in existence so we don't need a semaphore to
protect it.  Emphasize this by *not* returning a pointer to
the referenced operation.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>