acm: Return ENODEV instead of EINVAL when trying to open ACM device.

This is required, otherwise a user will get a EINVAL while opening a
non-existing device, instead of ENODEV.

This is what I get with this patch applied now instead of an "Invalid
argument".

  cascardo@vespa:~$ cat /dev/ttyACM0
  cat: /dev/ttyACM0: No such device
  cascardo@vespa:~$
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/usb/class/cdc-acm.c

@@ -550,7 +550,7 @@ static void acm_waker(struct work_struct *waker)
 static int acm_tty_open(struct tty_struct *tty, struct file *filp)
 {
 	struct acm *acm;
-	int rv = -EINVAL;
+	int rv = -ENODEV;
 	int i;
 	dbg("Entering acm_tty_open.");