ASoC: rt5514-spi: Let the buf_size to align with period_bytes

The patch lets the buf_size to align with period_bytes to prevent the
buffer reading over the real size of the DSP buffer and also avoid to
calculate the wrong size of remaining data.

Signed-off-by: Oder Chiou <oder_chiou@realtek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>