iio: imu: inv_mpu6050: stabilized timestamping in interrupt

Use IRQ ONESHOT flag to ensure the timestamp is not updated in the
hard handler during the thread handler. And use a fixed value of 1
sample that correspond to this first timestamp.

This way we can ensure the timestamp is always corresponding to the
value used by the timestamping mechanism. Otherwise, it is possible
that between FIFO count read and FIFO processing the timestamp is
overwritten in the hard handler.

Fixes: 111e1abd

 ("iio: imu: inv_mpu6050: use the common inv_sensors timestamp module")
Cc: stable@vger.kernel.org
Signed-off-by: Jean-Baptiste Maneyrol <jean-baptiste.maneyrol@tdk.com>
Link: https://lore.kernel.org/r/20240527150117.608792-1-inv.git-commit@tdk.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>