iio: hid-sensors: make hid_sensor_get_reporting_interval static

This fixes the following sparse warning:

  CHECK   drivers/iio/common/hid-sensors/hid-sensor-attributes.c
drivers/iio/common/hid-sensors/hid-sensor-attributes.c:346:5: warning:
symbol 'hid_sensor_get_reporting_interval' was not declared. Should it be static?
Signed-off-by: NDaniel Baluta <daniel.baluta@intel.com>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

drivers/iio/common/hid-sensors/hid-sensor-attributes.c

@@ -343,6 +343,7 @@ int hid_sensor_format_scale(u32 usage_id,
 }
 EXPORT_SYMBOL(hid_sensor_format_scale);
 
+static
 int hid_sensor_get_reporting_interval(struct hid_sensor_hub_device *hsdev,
 					u32 usage_id,
 					struct hid_sensor_common *st)