ASoC: Fix register cache sync register_writable WARN_ONs

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: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>