sound: ASoC: Fix WM8731 SPI write

Ensure wm8731_spi_write byte order is consistent regardless of
endianess.

Signed-off-by: Alan Horstmann <gineera@aspect135.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>