iio: imu: inv_mpu6050: read the full fifo when processing data

When processing data read the full fifo data in 1 time. If there
are several samples in the FIFO, it means we are experiencing
system delay. In this case, it is better to read all data with 1
bus access than to add additional latency by doing several ones.

This requires to use a bigger buffer depending on chip FIFO size
and do an additional local data copy before sending. But the cost
is minimal and behavior is still better like this under system
heavy load.

Signed-off-by: Jean-Baptiste Maneyrol <jean-baptiste.maneyrol@tdk.com>
Link: https://lore.kernel.org/r/20230623082924.283967-1-inv.git-commit@tdk.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>