ASoC: soc-cache: Use BUG_ON() for unsupported hw_read() calls

Instead of dereferencing a NULL function pointer and falling apart
use BUG_ON() for any unimplemented hw_read() calls.

Signed-off-by: Dimitris Papastamos <dp@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

diff --cc sound/soc/soc-cache.c
Simple merge