ASoC: Convert WM8900 to do more work at I2C probe time

Redo the instantiation of the WM8900 to do most of the initialisation
work when the I2C driver probes rather than when the ASoC device is
instantiated, registering the codec with the ASoC core when done.

Also move all dynamic allocations into a single kmalloc() to simplify
error handling and rename the I2C driver to make output more sensible.

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