media: bcm2835-unicam: Retain packing information on G_FMT

The change to retrieve the pixel format always on g_fmt didn't
check whether the native or unpacked version of the format
had been requested, and always returned the packed one.
Correct this so that the packing setting is retained whereever
possible.

Fixes "9d59e89e

 media: bcm2835-unicam: Re-fetch mbus code from subdev
on a g_fmt call"

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>