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: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>