Staging: iio: adc: fix indent on break statement

Fix indent warning when building with gcc 6:
drivers/staging/iio/adc/ad7192.c:239:4: warning: statement is indented
  as if it were guarded by... [-Wmisleading-indentation]
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>