ASoC: fix ak4104 register array access

Don't touch the variable 'reg' to construct the value for the actual SPI
transport. This variable is again used to access the driver's register
cache, and so random memory is overwritten.
Compute the value in-place instead.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Cc: stable@kernel.org
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>