iio: buffer-dma: Use round_down() instead of rounddown()

iio: buffer-dma: Use round_down() instead of rounddown()

We know that the buffer's alignment will always be a power of two;
therefore, we can use the faster round_down() macro.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Link: https://lore.kernel.org/r/20211115141925.60164-4-paul@crapouillou.net
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>