greybus: move gbuf initialization to caller

Change greybus_alloc_gbuf() so all it does is allocate the gbuf data
structure.  Move all of the initialization of the gbuf structure in
the caller.  Do the inverse in the caller prior to freeing the gbuf
structure via greybus_free_gbuf().  Use a null gbuf->transfer_buffer
pointer rather than a null gbuf pointer to indicate an unused gbuf.

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