iio: accel: make array init_data static to reduce code size

Making the const array init_data static rather having it on the
stack saves us a couple hundreds of bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
   3175	    848	      0	   4023	    fb7	drivers/iio/accel/da311.o

After:
   text	   data	    bss	    dec	    hex	filename
   2860	    936	      0	   3796	    ed4	drivers/iio/accel/da311.o

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>