staging: greybus: audio: Cleanup junk codec registers

Dummy codec register were initially added while populating dummy codec
mixer controls until module topology parser was available. Now, these
dummy registers are nowhere used and thus can be safely removed.

Since ASoC framework requires a valid callback for both read & write
register APIS, currently empty placeholders are kept to avoid panic.

Later, register mapping logic can be defined:
1. Assuming fixed number of maximum modules connected and register bits
corresponds to basic info of each module OR
2. With a logic to dynamically grow register_cache_size based on codec
modules added/removed.

Signed-off-by: Vaibhav Agarwal <vaibhav.agarwal@linaro.org>
Signed-off-by: Vaibhav Agarwal <vaibhav.sr@gmail.com>
Acked-by: Mark Greer <mgreer@animalcreek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>