greybus: define GB_OP_NONEXISTENT

The i2c protocol needs a way to indicate an i2c device doesn't exist
(which is not necessarily an error).  Define GB_OP_NONEXISTENT to
indicate this, and updating the status<->errno mapping functions
accordingly.

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