staging: iio: light: tsl2x7x constify attribute_group structures

As the event_attrs field of iio_info structures is constant, so these
attribute_group structures can also be declared constant.

File size before:
   text	   data	    bss	    dec	    hex	filename
  15064	   1528	      0	  16592	   40d0
drivers/staging/iio/light/tsl2x7x_core.o

File size after:
   text	   data	    bss	    dec	    hex	filename
  15192	   1400	      0	  16592	   40d0
drivers/staging/iio/light/tsl2x7x_core.o

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