greybus: spi: fix message transfer over greybus

The actual implementation of transfer_one_message have problems with
some cases in the possible transfer options. We try to maximize the
number of spi transfers in one greybus operation and need to save state
until the full message is dispatch over greybus.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Tested-by: Philip Yang <philipy@bsquare.com>
Tested-by: Axel Haslam <ahaslam@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>