greybus: firmware: Add 'status' byte to backend fw version response

The backend processor may not be ready to return the version of firmware
it is running by the time AP requests for it. The greybus specification
is updated to return 1-byte 'status' to return the error type, RETRY is
one of them.

This patch implements that in greybus now.

Note that the version structure was common across interface and backend
version requests earlier, but that is changing as well.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>