iio: frequency: Remove unnecessary braces around single statement block

This patch fixes the following checkpatch.pl warning:
WARNING: braces {} are not necessary for single statement blocks

Signed-off-by: Roberta Dobrescu <roberta.dobrescu@gmail.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Reviewed-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>