pxa2xx-ac97: switch AC unit to correct state before probing

If AC97 unit is in partially enabled state, early request_irq can trigger
IRQ storm or even full hang up. Workaround this by forcibly switching ACLINK off
at the start of the probe.

Signed-off-by: Dmitry Baryshkov <dbaryshkov@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>