iio:adc:at91-sama5d2: make array startup_lookup static to reduce code size

iio:adc:at91-sama5d2: make array startup_lookup static to reduce code size

Making the const array startup_lookup static rather having it on the
stack saves 69 bytes. Add in missing int specifier to clean up a
checkpatch warning.

Before:
   text	   data	    bss	    dec	    hex	filename
  10297	   2800	    128	  13225	   33a9	drivers/iio/adc/at91-sama5d2_adc.o

After:
   text	   data	    bss	    dec	    hex	filename
  10140	   2888	    128	  13156	   3364	drivers/iio/adc/at91-sama5d2_adc.o

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