[media] media: Init the reserved fields of struct media_link_desc

struct media_link_desc is copy_to_user'ed as the return value of
MEDIA_IOC_ENUM_LINKS. When copying, the driver is omitting to initialise
the reserved fields.  This commit fixes that by initialising the
reserved fields to 0.

CRs-Fixed: 570757
Change-Id: I230e2666c0845cc36399518a0f2c94db664382d1
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>