iio: invensense: fix timestamp glitches when switching frequency

When a sensor is running and there is a FIFO frequency change due to
another sensor turned on/off, there are glitches on timestamp. Fix that
by using only interrupt timestamp when there is the corresponding sensor
data in the FIFO.

Delete FIFO period handling and simplify internal functions.

Update integration inside inv_mpu6050 and inv_icm42600 drivers.

Fixes: 0ecc363c

 ("iio: make invensense timestamp module generic")
Cc: Stable@vger.kernel.org
Signed-off-by: Jean-Baptiste Maneyrol <jean-baptiste.maneyrol@tdk.com>
Link: https://lore.kernel.org/r/20240426094835.138389-1-inv.git-commit@tdk.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>