ASoC: Rename WM8915 to WM8996

For marketing reasons the part will be called WM8996. In order to avoid
user confusion rename the driver to reflect this.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Kukjin Kim <kgene.kim@samsung.com>
Acked-by: Liam Girdwood <lrg@ti.com>