media: rtl2832_sdr/vivid/airspy/hackrf/msi2500: drop memset of fmt.sdr.reserved

The V4L2 core already zeroes fmt.sdr.reserved, so there is no need for
drivers to do the same.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>