media: atomisp: Perform a single memset() for union

There is no need to memset() both dvs_grid_info and dvs_stat_grid_info
separately: they are part of the same union. Instead, just zero the
union itself. This additionally avoids a false positive (due to a
gcc optimization bug[1]) when building with the coming FORTIFY_SOURCE
improvements:

In function 'fortify_memset_chk',
    inlined from 'sh_css_pipe_get_grid_info' at drivers/staging/media/atomisp/pci/sh_css.c:5186:3,
    inlined from 'ia_css_stream_create' at drivers/staging/media/atomisp/pci/sh_css.c:9442:10:
./include/linux/fortify-string.h:199:4: warning: call to '__write_overflow_field' declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Wattribute-warning]
  199 |    __write_overflow_field();
      |    ^~~~~~~~~~~~~~~~~~~~~~~~

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101419

Link: https://lore.kernel.org/linux-media/20210711145101.1434065-1-keescook@chromium.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

drivers/staging/media/atomisp/pci/sh_css.c

@@ -5179,10 +5179,7 @@ sh_css_pipe_get_grid_info(struct ia_css_pipe *pipe,
 		ia_css_binary_dvs_grid_info(binary, info, pipe);
 		ia_css_binary_dvs_stat_grid_info(binary, info, pipe);
 	} else {
-		memset(&info->dvs_grid.dvs_grid_info, 0,
-		       sizeof(info->dvs_grid.dvs_grid_info));
-		memset(&info->dvs_grid.dvs_stat_grid_info, 0,
-		       sizeof(info->dvs_grid.dvs_stat_grid_info));
+		memset(&info->dvs_grid, 0, sizeof(info->dvs_grid));
 	}
 
 	if (binary) {