media: rcar_drif: fix a memory disclosure

"f->fmt.sdr.reserved" is uninitialized. As other peer drivers
like msi2500 and airspy do, the fix initializes it to avoid
memory disclosures.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
