iio: imu: inv_mpu6050: skip first sample when gyro is on

Implement generic skip first samples mechanism and use it to
filter out first sample when gyro is on.

The problem for these chips is that the first sample of the gyro
is out of specs, because gyro is not completely stabilized. To
ensure all data are within sensor specs, we just skip the first
sample when turning gyro on.

Signed-off-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>