tools:iio:iio_utils: mark private function static

Functions _write_sysfs_int() and _write_sysfs_string() are supposed to
be called only by public wrappers, so make them static.

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>