Currently the condition for these WARN_ONs is reversed and they are placed
before the actual check whether we are going to write to that register. So if
the codec implements the register_writable callback we'll get a warning for each
writable register when syncing the register cache.

While we are at it change the check to use snd_soc_codec_writable_register
instead of open-coding it.
Signed-off-by: NLars-Peter Clausen <lars@metafoo.de>
Acked-by: NLiam Girdwood <lrg@ti.com>
Signed-off-by: NMark Brown <broonie@opensource.wolfsonmicro.com>