imu drivers: do not set on-chip filter based on driver filter setting

The description of IMU_GYRO_CUTOFF was incorrectly saying that it only
affects the driver filtering, but in fact it also set the on-chip filter
to the next higher supported value. This patch fixes that.

And because the IMU_GYRO_CUTOFF and not the IMU_ACCEL_CUTOFF was used for
the on-chip filter, after #9070 which sets the default gyro filter to 80,
we were effectively using a dlpf of 98 Hz.
For this reason this patch changes the on-chip cutoff frequency to 98 as
well, so that the overall default behavior is unchanged.