greybus: convert drivers to use connection->private set/get

This converts all drivers to use the gb_connection_get_data() and
gb_connection_set_data() functions to make it a bit more explicit as to
what is going on.

Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>