soc: make snd_event stub functions as static inline

Modify snd_event stub functions prototype as static inline
to make sure that there are no multiple definition errors
in the case of static compilation.

Change-Id: I0848be946b332df22c045b8e5fed4c39b7ee0fda
Signed-off-by: Vidyakumar Athota <vathota@codeaurora.org>