drivers: staging: audio: Fix the missing header file for helper file

This patch fixes the warning reported for missing prototypes due to
missing header file. Also, it includes changes to remove
unused_but_set_variables.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Vaibhav Agarwal <vaibhav.sr@gmail.com>
Link: https://lore.kernel.org/r/6341df9b0b5985047af0bbbc8e136481ac515b25.1594290158.git.vaibhav.sr@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>