ASoC: rt286: Fix null pointer issue

To make the interrupt safe if it happens to be called before the card is
ready, we use regmap read/write in the interrupt handler. Also, we try to
prevent the interrupt happen before the card is ready by enabling codec's
IRQ in the ASoC probe.

Signed-off-by: Bard Liao <bardliao@realtek.com>
Signed-off-by: Mark Brown <broonie@linaro.org>