staging: iio: frequency: add new line after declarations

This fixes the checkpatch.pl warning:
WARNING: Missing a blank line after declarations

Signed-off-by: Roxana Blaj <roxanagabriela10@gmail.com>
Acked-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>