iio: imu: inv_mpu6050: do not flush fifo when iio buffer is full

There is no need to flush fifo and loose all data when the iio
buffer is full. Just drop the data by ignoring the error as
commonly done in other drivers.

Signed-off-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Reviewed-by: Martin Kelly <martin@martingkelly.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>