greybus: i2c-gb: fix bad message size in gb_i2c

The data_in_size variable was set to 1 for the status byte.
But now, the status byte has move to header. Then, the status byte
is "allocated" twice and cause bad message size error.

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