soundwire: add module_sdw_driver helper macro

This Helper macro is for Soundwire drivers which do not do anything special in
module init/exit. This eliminates a lot of boilerplate. Each module may only
use this macro once, and calling it replaces module_init() and module_exit()

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>