ASoC: wm8960: add support for big-endian audio samples

PowerPC ASoC drivers frequently use the _BE variants of the SNDRV_PCM_FORMAT
macros, so we need to look for those as well.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>