iio: buffer-dma: Use ARRAY_SIZE in for loop range

Use the ARRAY_SIZE macro in the for loops that access queue->fileio.blocks.
Macro is already used in a couple of places where this access occurs,
but range was hardcoded in these locations.

Signed-off-by: Phil Reid <preid@electromag.com.au>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>