ASoC: wm_adsp: Improve round to next 4-byte boundary

Whilst the existing code does correctly round to the next 4-byte boundary
it does so rather inefficiently. This patch changes the rounding to be
simpler and more efficient.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>