iio: dummy: iio_simple_dummy: Add newline after function-end

Fixes the following checkpatch.pl warning(s):

 CHECK: Please use a blank line after function/struct/union/enum declarations
 #46: FILE: drivers/iio/dummy/iio_simple_dummy.c:690:
  }
 +/*
 total: 0 errors, 0 warnings, 1 checks, 22 lines checked

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>