um: Remove unneeded variable 'ret'

Fix the following coccicheck warning:
./arch/um/drivers/hostaudio_kern.c:125:10-14: Unneeded variable: "mask".
Return "0" on line 131

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Richard Weinberger <richard@nod.at>