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>

arch/um/drivers/hostaudio_kern.c

@@ -122,13 +122,11 @@ static ssize_t hostaudio_write(struct file *file, const char __user *buffer,
 static __poll_t hostaudio_poll(struct file *file,
 				struct poll_table_struct *wait)
 {
-	__poll_t mask = 0;
-
 #ifdef DEBUG
 	printk(KERN_DEBUG "hostaudio: poll called (unimplemented)\n");
 #endif
 
-	return mask;
+	return 0;
 }
 
 static long hostaudio_ioctl(struct file *file,