ASoC: wm8962: Fix null pointer pdata access in I2C probe()

When using DT binding to pass private data, there would be Kernel panic
occuring due to NULL pointer access in wm8962_i2c_probe(). Thus fix it.

Signed-off-by: Nicolin Chen <b42378@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>