staging:iio:accel:sca3000 Fix clearing of flag + setting of size of scan.

Not clearing the stuff_to_read flag can lead to a false flag being set
on restarting the buffer if the data was not all read the previous time.

The size of the scan is needed to ensure the function
iio_buffer_read_first_n_outer actually tries to read the data.

This stuff has been broken for some time so not stable material.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>