ASoC: Remove byte swap in 4x12 SPI write

snd_soc_4_12_spi_write() contains a byte swap. Since this code was written
for an Analog CODEC on a Blackfin reference board it appears that this is
done because while Blackfin is little endian the CODEC is big endian (as
are most CODECs).

Push this up into the generic 4x12 write function and use cpu_to_be16() to
do the byte swap so things are more regular and things work on both CPU
endiannesses.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@ti.com>