media: atomisp: get rid of wrong stddef.h include

The places at atomisp.h that use stddef.h are wrong: the
types it needs are actually defined at linux/types.h. Also,
it causes lots of smatch warnings due to the redefinition of
ofsetof() macro:

/opt/gcc-7.1.0/x86/lib/gcc/x86_64-pc-linux-gnu/7.1.0/include/stddef.h:417:9: warning: preprocessor token offsetof redefined
./include/linux/stddef.h:16:9: this was the original definition

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi>