drivers: media: cfe: Increase default size of embedded buffer

Increase the size of the default embedded buffer to 16k. This is done to
match what is advertised by the IMX219 driver and workaround a problem
where the embedded stream is not actually used. Without full streams API
support, the media pipeline validation will fail in these circumstances.

Signed-off-by: Naushir Patuck <naush@raspberrypi.com>