ASoC: wm8903: Make interrupt handler use regmap directly

There's no urgent need for the interrupt handler to use the ASoC I/O
functions and it'll support a further move in where we request the
interrupt so call the regmap APIs directly.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Tested-by: Stephen Warren <swarren@wwwdotorg.org>