greybus: use null gbuf->transfer_buffer

Make sure gbuf->transfer_buffer gets reset to NULL when the buffer
is freed.  We can leverage that to do a little extra error checking.
We'll also use a null transfer buffer in the next patch to indicate
an unused gbuf.

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