ASoC: Fix WM8978/migor driver name conflict

Standardise on 'wm8978' as the name for the CODEC.

Reported-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>