sound: ASoC: Add WM8750 SPI support

Implement SPI support for WM8750, cut'n'pasting from the support for
WM8731 contributed by Cliff Cai and Alan Horstmann since the wire format
is the same for both codecs.

Also fix a cut'n'pasted comment in the I2C side of the driver (which was
clearly written in the same way) while we're at it.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>