iio: imu: st_lsm6dsx: discard samples during filters settling time

iio: imu: st_lsm6dsx: discard samples during filters settling time

During digital filters settling time the driver is expected to drop
samples since they can be corrupted. Introduce the capability to drop
a given number of samples according to the configured ODR.
Add sample_to_discard for LSM6DSM-like sensors since new generation
devices (e.g. LSM6DSO) support DRDY mask where corrupted samples are
masked in hw with values greather than 0x7ffd so the driver can easily
discard them.
I have not added sample_to_discard support for LSM6DS3 or LSM6DS3H since
I do not have any sample for testing at the moment.

Reported-by: Philippe De Muyter <phdm@macqel.be>
Tested-by: Philippe De Muyter <phdm@macqel.be>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/21dcd94935c147ef9b1da4984b3da6264ee9609e.1677496295.git.lorenzo@kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>