char/mwave: turn tp3780I_Cleanup() into void function

char/mwave: turn tp3780I_Cleanup() into void function

This function always return '0' and no callers use the return value.
So make it a void function.

This eliminates the following coccicheck warning:
./drivers/char/mwave/tp3780i.c:182:5-11: Unneeded variable: "retval".
Return "0" on line 187

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Link: https://lore.kernel.org/r/1615366834-20545-1-git-send-email-yang.lee@linux.alibaba.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>