Staging: iio: adc: Fix else after return warning

This patch fixes checkpatch.pl warning:
WARNING: else is not generally useful after a break or return

Signed-off-by: Darshana Padmadas <darshanapadmadas@gmail.com>
Acked-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>