media: i2c: ds90ub960: Allow FPD-Link async mode

Allow using FPD-Link in async mode. The driver handles it correctly, but
the mode was blocked at probe time as there wasn't HW to test this with.
Now the mode has been tested, and it works.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>