test_cros_ec_gyro_iio_abi: Add a condition for frequency attribute

Frequency sysfs attribute have been replaced by the
standard sampling_frequency after 5.4.0. We need to check
the version of running kernel to decide if we must check it.

Signed-off-by: Fabien Lahoudere <fabien.lahoudere@collabora.com>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>