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>