ASoC: Move most WM8996 resource acquisition to I2C probe

Now that the WM8996 driver is using the regmap API for register I/O we no
longer need the ASoC card to be active in order to interact with the chip.
In order to be more idiomatic for Linux move most of the existing probe()
function out into the I2C probe() function prior to registration with ASoC.

The IRQ and GPIO init will be moved separately as these are slightly more
involved.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>