media: isp: fix a warning about a wrong struct initializer

As sparse complains:
	drivers/media/platform/omap3isp/isp.c:303:39: warning: Using plain integer as NULL pointer

when a struct is initialized with { 0 }, actually the first
element of the struct is initialized with zeros, initializing the
other elements recursively. That can even generate gcc warnings
on nested structs.

So, instead, use the gcc-specific syntax for that (with is used
broadly inside the Kernel), initializing it with {};

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>