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>