iio: adc: ad799x: constify attribute_group structures

Check for attribute_group structures that are only stored in the
event_attrs filed of iio_info structure. As the event_attrs field of
iio_info structures is constant, so these attribute_group structures can
also be declared constant.
Done using coccinelle:

@r1 disable optional_qualifier @
identifier i;
position p;
@@
static struct attribute_group i@p = {...};

@ok1@
identifier r1.i;
position p;
struct iio_info x;
@@
x.event_attrs=&i@p;

@bad@
position p!={r1.p,ok1.p};
identifier r1.i;
@@
i@p

@depends on !bad disable optional_qualifier@
identifier r1.i;
@@
static
+const
struct attribute_group i={...};

@depends on !bad disable optional_qualifier@
identifier r1.i;
@@
+const
struct attribute_group i;

File size before:
   text    data     bss     dec     hex filename
  26051     464       0   26515    6793 drivers/iio/adc/ad799x.o

File size after:
   text	   data	    bss	    dec	    hex	filename
  26115	    400	      0	  26515	   6793	drivers/iio/adc/ad799x.o

Signed-off-by: simran singhal <singhalsimran0@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>