usb: cdc-acm: handle unlinked urb in acm read callback

commit 19454462acb1bdef80542061bdc9b410e4ed1ff6 upstream.

In current acm driver, the bulk-in callback function ignores the
URBs unlinked in usb core.

This causes unexpected data loss in some cases. For example,
runtime suspend entry will unlinked all urbs and set urb->status
to -ENOENT even those urbs might have data not processed yet.
Hence, data loss occurs.

This patch lets bulk-in callback function handle unlinked urbs
to avoid data loss.

Signed-off-by: Tang Jian Qiang <jianqiang.tang@intel.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Acked-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[ luis: backported to 3.16:
  - use 'urb->status' instead of local variable 'status'
  - adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/usb/class/cdc-acm.c

diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
index f2dc462..9cda900 100644
--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c
@@ -427,7 +427,8 @@
 	if (urb->status) {
 		dev_dbg(&acm->data->dev, "%s - non-zero urb status: %d\n",
 							__func__, urb->status);
-		return;
+		if ((urb->status != -ENOENT) || (urb->actual_length == 0))
+			return;
 	}
 
 	usb_mark_last_busy(acm->dev);