media: imx-media-internal-sd: Use empty initializer

When building with W=1 the following warning shows up:

drivers/staging/media/imx/imx-media-internal-sd.c:274:49: warning: Using plain integer as NULL pointer

Fix this problem by using an empty initializer, which guarantees that all
struct members are zero-cleared.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Steve Longerbeam <steve_longerbeam@mentor.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>