iio: imu: inv_mpu6050: make loop a do-while

Prior to this loop, we check if fifo_count < bytes_per_datum and bail if
so. This means that when we hit the loop, we know that fifo_count >=
bytes_per_datum, so the check is unneeded and we can turn the loop into
a do-while for a slight performance improvement.

Signed-off-by: Martin Kelly <mkelly@xevo.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>