media: staging: atomisp: fix for sparse "using plain integer as NULL pointer" warnings

The "address" member of struct ia_css_host_data is a pointer-to-char,
so define default as NULL.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>