media: media-device.c: zero reserved fields

MEDIA_IOC_SETUP_LINK didn't zero the reserved field of the media_link_desc
struct. Do so in media_device_setup_link().

MEDIA_IOC_ENUM_LINKS didn't zero the reserved field of the media_links_enum
struct. Do so in media_device_enum_links().

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>