greybus: record the host device in a gbuf

The only thing we now use the gbuf->operation pointer for is
to get access to its connection's host device.

Record the host device pointer directly in the gbuf, rather
than keeping a pointer to the operation.

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