ASoC: wm8523: Set bclk ratio for master mode

When running in master mode the bclk divider must be configured to generate a
sane bitclock. Pick the smallest fs multiplicator, which can hold all
transmitted bits.

Signed-off-by: Julian Scheel <julian@jusst.de>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>