iio:imu:inv_mpu6050 Fix dma and ts alignment and data leak issues.

This case is a bit different to the rest of the series.  The driver
was doing a regmap_bulk_read into a buffer that wasn't dma safe
as it was on the stack with no guarantee of it being in a cacheline
on it's own.   Fixing that also dealt with the data leak and
alignment issues that Lars-Peter pointed out.

Also removed some unaligned handling as we are now aligned.

Fixes tag is for the dma safe buffer issue. Potentially we would
need to backport timestamp alignment futher but that is a totally
different patch.

Fixes: fd64df16 ("iio: imu: inv_mpu6050: Add SPI support for MPU6000")
Reported-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Cc: <Stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200722155103.979802-18-jic23@kernel.org