media: i2c: ds90ub960: Rename RXPORT_MODE_CSI2_ASYNC to RXPORT_MODE_CSI2_NONSYNC

FPD-Link has an operating mode that used to be called "asynchronous" in
the hardware documentation, but that has been changed to non-synchronous
already quite a while back. The ub960 driver still had one instance of
the old naming, so let's rename it.

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