iio: hid-sensors: Set default unit of measure for report interval

For PROP_REPORT_INTERVAL, the spec does not mandate the presence
of the report interval unit in the feature report and expects the
default unit of measure to be used as milliseconds.

Currently, when the unit is not present, it gets set as zero leading
to issues in sampling frequency.

This patch sets the unit of measure to the default unit if it
is not defined by firmware.

Signed-off-by: Archana Patni <archana.patni@intel.com>
Signed-off-by: Subramony Sesha <subramony.sesha@intel.com>
Reviewed-by: Srinivas Pandruvada <srinivas.pandruvada@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>