ASoC: rt1015: modify pre-divider for sysclk

Modify pre-divider for system clock.

Signed-off-by: Jack Yu <jack.yu@realtek.com>
Link: https://lore.kernel.org/r/20200303025913.24499-1-jack.yu@realtek.com
Signed-off-by: Mark Brown <broonie@kernel.org>